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Boat_upstream
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I like to begin with a "word equation." We can write:A boat travelled upstream a distance of 90 miles at an average speed of (v-3) miles per hour and then travelled downstream at an average speed of (V+3) miles per hour. If the trip upstream took half an hour longer than the trip downstream, how many hours did it take the boat to travel downstream?
A) 2.5
B) 2.4
C) 2.3
D) 2.2
E) 2.1
travel time upstream = travel time downstream + 1/2
Time = distance/rate
So, we can replace elements in our word equation to get:
90/(v-3) = 90/(v+3) + 1/2
Now solve for v (lots of work here)
.
.
.
v = 33
So, travel time downstream = 90/(v+3)
= 90/(33+3)
= 90/36
= 5/2
= 2 1/2 hours
Cheers,
Brent
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Hi mukherjee.tanuj3,
This is a layered story-problem and takes a lot of effort to solve using a traditional "math approach" (Brent notes it in his explanation: "lots of work here"). Here's how you can solve it with a bit of logic and TESTing THE ANSWERS:
From the prompt, we can create 2 equations:
D = R x T
90 = (V-3)(T + 1/2)
90 = (V+3)(T)
We're asked for the value of T.
From the prompt, I find it interesting that the distance is a nice, round number (90).... because when looking at the answer choices, most of them are NOT nice decimals. When multiplying two values together (as we do in BOTH equations), if you end up with a round number, chances are that either....
1) both numbers are round numbers
2) one of the numbers includess a nice fraction (e.g. 1/2) which can be multiplied and the end result will be a round number.
This gets me thinking that 2.5 is probably the answer, but I still have to prove it....I'm going to plug in THAT value for T and see what happens to the 2 equations....
90 = (V-3)(3)
90 = (V+3)(2.5)
30 = (V-3)
36 = (V+3)
33 = V
33 = V
Notice how both values of V are THE SAME? That means that we have the solution. V=33 and T=2.5
Final Answer:A
GMAT assassins aren't born, they're made,
Rich
This is a layered story-problem and takes a lot of effort to solve using a traditional "math approach" (Brent notes it in his explanation: "lots of work here"). Here's how you can solve it with a bit of logic and TESTing THE ANSWERS:
From the prompt, we can create 2 equations:
D = R x T
90 = (V-3)(T + 1/2)
90 = (V+3)(T)
We're asked for the value of T.
From the prompt, I find it interesting that the distance is a nice, round number (90).... because when looking at the answer choices, most of them are NOT nice decimals. When multiplying two values together (as we do in BOTH equations), if you end up with a round number, chances are that either....
1) both numbers are round numbers
2) one of the numbers includess a nice fraction (e.g. 1/2) which can be multiplied and the end result will be a round number.
This gets me thinking that 2.5 is probably the answer, but I still have to prove it....I'm going to plug in THAT value for T and see what happens to the 2 equations....
90 = (V-3)(3)
90 = (V+3)(2.5)
30 = (V-3)
36 = (V+3)
33 = V
33 = V
Notice how both values of V are THE SAME? That means that we have the solution. V=33 and T=2.5
Final Answer:A
GMAT assassins aren't born, they're made,
Rich
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GMATGuruNY
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We can PLUG IN THE ANSWERS, which represent the number of hours that the boat took to travel downstream.A boat traveled upstream a distance of 90 miles at an average speed of
(v-3) miles per hour and then traveled the same distance downstream at an
average speed of (v+3) miles per hour. If the trip upstream took half an
hour longer than the trip downstream, how many hours did it take the boat
to travel downstream?
a) 2.5
b) 2.4
c) 2.3
d) 2.2
e) 2.1
The most likely answer choice is A -- the only option that divides evenly into 90.
Answer choice A: 2.5 hours to travel downstream.
Rate downstream = d/t = 90/(2.5) = 36 miles per hour.
Thus, v+3 = 36, implying that v=33.
Rate upstream = v-3 = 33-3 = 30 miles per hour.
Time upstream = d/r = 90/30 = 3 hours.
Time upstream - time downstream = 3-2.5 = .5.
Success!
The correct answer is A.
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I have worked with students based in the US, Australia, Taiwan, China, Tajikistan, Kuwait, Saudi Arabia -- a long list of countries.
My students have been admitted to HBS, CBS, Tuck, Yale, Stern, Fuqua -- a long list of top programs.
As a tutor, I don't simply teach you how I would approach problems.
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Amrabdelnaby
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Hi Brent,
I noticed while solving this question that it takes - if you know what you are doing - between 3-4 minutes to solve this question for V and do all the calculations.
I was wondering if i encounter such a question in the assessment day if it is worthy to spend such time on it and i also wanted to ask you what is the level of this question?
i think its 700+ right?
I noticed while solving this question that it takes - if you know what you are doing - between 3-4 minutes to solve this question for V and do all the calculations.
I was wondering if i encounter such a question in the assessment day if it is worthy to spend such time on it and i also wanted to ask you what is the level of this question?
i think its 700+ right?
Brent@GMATPrepNow wrote:I like to begin with a "word equation." We can write:A boat travelled upstream a distance of 90 miles at an average speed of (v-3) miles per hour and then travelled downstream at an average speed of (V+3) miles per hour. If the trip upstream took half an hour longer than the trip downstream, how many hours did it take the boat to travel downstream?
A) 2.5
B) 2.4
C) 2.3
D) 2.2
E) 2.1
travel time upstream = travel time downstream + 1/2
Time = distance/rate
So, we can replace elements in our word equation to get:
90/(v-3) = 90/(v+3) + 1/2
Now solve for v (lots of work here)
.
.
.
v = 33
So, travel time downstream = 90/(v+3)
= 90/(33+3)
= 90/36
= 5/2
= 2 1/2 hours
Cheers,
Brent
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Hi Amrabdelnaby,
When you say that it took 3-4 minutes of work, what were you doing exactly during those 3-4 minutes? While this question is 'thick' and takes longer than average for most Test Takers to solve, it might be that 'your way' of tackling this question is why it took so long to solve. If you TEST THE ANSWERS and think about the Number Properties involved, then you likely would have answered it much faster.
If you're facing overall pacing problems during the Quant section of your CATs, and you can't answer these types of questions faster than you currently are, then you might need to dump them so that you can spend more time on the rest of the section.
GMAT assassins aren't born, they're made,
Rich
When you say that it took 3-4 minutes of work, what were you doing exactly during those 3-4 minutes? While this question is 'thick' and takes longer than average for most Test Takers to solve, it might be that 'your way' of tackling this question is why it took so long to solve. If you TEST THE ANSWERS and think about the Number Properties involved, then you likely would have answered it much faster.
If you're facing overall pacing problems during the Quant section of your CATs, and you can't answer these types of questions faster than you currently are, then you might need to dump them so that you can spend more time on the rest of the section.
GMAT assassins aren't born, they're made,
Rich
Hey Brent,Brent@GMATPrepNow wrote:[
Now solve for v (lots of work here)
.
.
.
v = 33
So, travel time downstream = 90/(v+3)
= 90/(33+3)
= 90/36
= 5/2
= 2 1/2 hours
Cheers,
Brent
When you say lots of work here, am I do it wrong?
90/(v-3) = 90/(v+3)+1/2
Find a common denom for the right side of equality so
90/(v-3)={180/2v+6}+{v+3/2v+6]
combine terms under same denom
90/(v-3) = (183+v)/(2v+6)
cross multiply
183v-549+v^2-3v=180v+540
v^2-9=0
(v-3) (v+3) = 0
v=3 or -3
I didn't get v=33
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Through the equation in green, your work is correct.prada wrote: When you say lots of work here, am I do it wrong?
90/(v-3) = 90/(v+3)+1/2
Find a common denom for the right side of equality so
90/(v-3)={180/2v+6}+{v+3/2v+6]
combine terms under same denom
90/(v-3) = (183+v)/(2v+6)
cross multiply
183v-549+v^2-3v=180v+540
Combining like terms, we get:
v² + 183v - 180v - 3v - 549 - 540 = 0
v² + 0 - 1089 = 0
v² - 1089 = 0
(v + 33)(v - 33) = 0.
v = ±33.
Since the rate cannot be negative, the only valid solution is v=33.
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I unlock the best way for YOU to solve problems.
For more information, please email me (Mitch Hunt) at [email protected].
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Followed here and elsewhere by over 1900 test-takers.
I have worked with students based in the US, Australia, Taiwan, China, Tajikistan, Kuwait, Saudi Arabia -- a long list of countries.
My students have been admitted to HBS, CBS, Tuck, Yale, Stern, Fuqua -- a long list of top programs.
As a tutor, I don't simply teach you how I would approach problems.
I unlock the best way for YOU to solve problems.
For more information, please email me (Mitch Hunt) at [email protected].
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Brent@GMATPrepNow
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Here's another upstream/downstream question to practice with - https://www.beatthegmat.com/speedboat-t40512.html
Cheers,
Brent
Cheers,
Brent
Brent Hanneson - Creator of GMATPrepNow.com
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Matt@VeritasPrep
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Multiply both sides by (v-3)(v+3)*2:prada wrote:90/(v-3) = 90/(v+3)+1/2
180(v+3) = 180(v-3) + (v-3)(v+3)
Subtract 180*(v+3) from both sides:
0 = 180(v-3) - 180(v+3) + (v-3)(v+3)
Factor:
0 = 180((v - 3) - (v + 3)) + (v - 3)(v + 3)
0 = 180(-6) + (v-3)(v+3)
0 = v² - 9 -1080
1089 = v²
v = ±33
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90/(v-3)-90/(v+3)=30/60(convert min into hrs) (Difference of time going up & down)
90(v+3)-90(v-3)/(v-3)(v+3)=1/2
simplify
90v+270-90v+270/v^2-9=1/2
540*2=v^2-9
1080+9=V^2
0r V=1089^1/2=33
time taken to come down=90/33+3= 90/36= 5/2=2.5hrs
90(v+3)-90(v-3)/(v-3)(v+3)=1/2
simplify
90v+270-90v+270/v^2-9=1/2
540*2=v^2-9
1080+9=V^2
0r V=1089^1/2=33
time taken to come down=90/33+3= 90/36= 5/2=2.5hrs
