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Maria and Perry

by j_shreyans » Wed Oct 01, 2014 4:01 am
Two musicians, Maria and Perry, work at independent constant rates to tune a warehouse full of instruments. If both musicians start at the same time and work at their normal rates, they will complete the job in 45 minutes. However, if Perry were to work at twice Maria's rate, they would take only 20 minutes. How long would it take Perry, working alone at his normal rate, to tune the warehouse full of instruments?

A)1 hr 20 min
B)1 hr 45 min
C)2 hr
D)2 hr 20 min
E)3 hr

Guys ,

Let Maria be M and Perry be P

So P+M = 1/45 mins

Given that if Perry were to work at twice Maria's rate

than P=2M

so M+2M=1/20

So M=1/60

Now next what??

Pls correct if i am wrong....
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by GMATinsight » Wed Oct 01, 2014 4:13 am
Answer: Option E
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by GMATGuruNY » Wed Oct 01, 2014 5:24 am
Two musicians, Maria and Perry, work at independent constant rates to tune a warehouse full of instruments. If both musicians start at the same time and work at their normal rates, they will complete the job in 45 minutes. However, if Perry were to work at twice Maria's rate, they would take only 20 minutes. How long would it take Perry, working alone at his normal rate, to tune the warehouse full of instruments?

A 1 hr 20 min
B 1 hr 45 min
C 2 hr
D2 hr 20 min
E 3 hr
Let the job = the LCM of 45 and 20 = 180 units.

Since Maria and Perry working at their normal rates take 45 minutes, the combined normal rate for M+P = w/t = 180/45 = 4 units per minute.

When Perry works at twice Maria's rate, the combined faster rate for P+M = 2M + M = 3M.
Since the time decreases to 20 minutes, the rate for 3M = w/t = 180/20 = 9 units per minute.
Since the rate for 3M = 9 units per minute, the rate for M alone = 3 units per minute.

P's rate alone = combined normal rate for P+M - M's rate = 4-3 = 1 unit per minute.
Thus:
Time for P alone = w/r = 180/1 = 180 minutes = 3 hours.

The correct answer is E.
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by Brent@GMATPrepNow » Wed Oct 01, 2014 6:11 am
j_shreyans wrote:Two musicians, Maria and Perry, work at independent constant rates to tune a warehouse full of instruments. If both musicians start at the same time and work at their normal rates, they will complete the job in 45 minutes. However, if Perry were to work at twice Maria's rate, they would take only 20 minutes. How long would it take Perry, working alone at his normal rate, to tune the warehouse full of instruments?

A)1 hr 20 min
B)1 hr 45 min
C)2 hr
D)2 hr 20 min
E)3 hr
One option is to assign a value to the total job.
Since the Least Common Multiple of 45 and 20 is 180, let's say that there are 180 instruments in the warehouse.

Let M = the number of instruments that Maria can tune PER MINUTE
Let P = the number of instruments that Perry can tune PER MINUTE

Both musicians working TOGETHER complete the job in 45 minutes
180/45 = 4
So, working TOGETHER, they can tune 4 instruments PER MINUTE
In other words, (Mary's rate) + (Perry's rate) = 4
We can write: M + P = 4

If Perry were to work at twice Maria's rate, they would take only 20 minutes.
180/20 = 9
So, in this scenario, they can tune 9 instruments PER MINUTE
In other words, (Mary's rate) + (Perry's rate) = 9
In this scenario, Perry's rate = 2M
So, we can write: M + 2M = 9
Simplify: 3M = 9
So, M = 3 (Maria can tune 3 instruments per minute)

Now that we know the value of M, we can use the equation M + P = 4 to conclude that P = 1
In other words, Perry can tune 1 instrument per minute

If there are 180 instruments to tune, it will take Perry 180 minutes to complete the job.

Answer: E

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by Brent@GMATPrepNow » Wed Oct 01, 2014 6:12 am
j_shreyans wrote:Two musicians, Maria and Perry, work at independent constant rates to tune a warehouse full of instruments. If both musicians start at the same time and work at their normal rates, they will complete the job in 45 minutes. However, if Perry were to work at twice Maria's rate, they would take only 20 minutes. How long would it take Perry, working alone at his normal rate, to tune the warehouse full of instruments?

A)1 hr 20 min
B)1 hr 45 min
C)2 hr
D)2 hr 20 min
E)3 hr
Another approach:

For work questions, there are two useful rules:

Rule #1: If a person can complete an entire job in k hours, then in one hour, the person can complete 1/k of the job
Example: If it takes Sue 5 hours to complete a job, then in one hour, she can complete 1/5 of the job. In other words, her work rate is 1/5 of the job per hour

Rule #2: If a person completes a/b of the job in one hour, then it will take b/a hours to complete the entire job
Example: If Sam can complete 1/8 of the job in one hour, then it will take him 8/1 hours to complete the job.
Likewise, if Joe can complete 2/3 of the job in one hour, then it will take him 3/2 hours to complete the job.

Let's use these rules to solve the question. . . .

Let M = the FRACTION of the total job that Maria can complete (working alone) in 1 MINUTE.
Let P = the FRACTION of the total job that Perry can complete (working alone) in 1 MINUTE

Both musicians working TOGETHER complete the job in 45 minutes
By Rule #1, we can conclude that, working together, Maria and Perry can complete 1/45 of the total job in 1 MINUTE
So, in 1 MINUTE, we can says that (Maria's contribution) + (Perry's contribution) = 1/45 of the total job
We can write: M + P = 1/45

If Perry were to work at twice Maria's rate, they would take only 20 minutes.
By Rule #1, we can conclude that, working together, Maria and Perry can complete 1/20 of the total job in 1 MINUTE
So, in 1 MINUTE, we can says that (Maria's contribution) + (Perry's contribution) = 1/20 of the total job
If Perry's rate is twice Maria's, then in 1 MINUTE, the fraction of the job that Perry can complete = 2M
So, we can write: M + 2M = 1/20
Simplify: 3M = 1/20
Solve: M = 1/60 (In 1 MINUTE, Maria can complete 1/60 of the job)

Now that we've solved for M, we can take the equation M + P = 1/45 and replace M with 1/60 to get: 1/60 + P = 1/45
Rewrite using common denominator: 3/180 + P = 4/180
Solve: P = 1/80
So, in 1 MINUTE, Perry can complete 1/180 of the job
By Rule #2, we can conclude that Perry can complete the ENTIRE job in 180 minutes.

Answer: E

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by Brent@GMATPrepNow » Wed Oct 01, 2014 6:20 am
j_shreyans wrote:Two musicians, Maria and Perry, work at independent constant rates to tune a warehouse full of instruments. If both musicians start at the same time and work at their normal rates, they will complete the job in 45 minutes. However, if Perry were to work at twice Maria's rate, they would take only 20 minutes. How long would it take Perry, working alone at his normal rate, to tune the warehouse full of instruments?

A)1 hr 20 min
B)1 hr 45 min
C)2 hr
D)2 hr 20 min
E)3 hr

Guys ,

Let Maria be M and Perry be P

So P+M = 1/45 mins

Given that if Perry were to work at twice Maria's rate

than P=2M

so M+2M=1/20

So M=1/60

Now next what??

Pls correct if i am wrong....
When assigning variables, you should be more precise than saying "Let Maria be M and Perry be P"
Does M = the RATE (instruments per minute/hour) at which Maria can tune instruments, or does M = the TIME it takes Maria to tune an instrument?
Your solution treats M as meaning BOTH of those.

What does M and P represent when you write P+M = 1/45 mins?
What do these variables mean when you write P = 2M?

Once you answer these questions, you'll see where you went wrong.

Cheers,
Brent
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by neha shekhawat » Sat Jan 21, 2017 9:48 am
Hi,
I got confused while solving this question.I can easily solve it by forming equations in terms of rate.But I tried to solve it by using equations in terms of time i.e let rate of maria= M and rate of perry = P
then
1/M + 1/P=45 equation (1)
and another equation 1/M + 1/2M=20 equation (2) by solving this I got 1/M=40/3
using this inserting value of 1/M in equation (1)
I got 1/P=95/3 which is totally different from answer that I got using equations in terms of rate.
Where am I going wrong?
why can't we solve this question using equations in terms of time.
please help
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by Matt@VeritasPrep » Wed Feb 01, 2017 5:27 pm
neha shekhawat wrote:Hi,
I got confused while solving this question.I can easily solve it by forming equations in terms of rate.But I tried to solve it by using equations in terms of time i.e let rate of maria= M and rate of perry = P
then
1/M + 1/P=45 equation (1)
and another equation 1/M + 1/2M=20 equation (2) by solving this I got 1/M=40/3
using this inserting value of 1/M in equation (1)
I got 1/P=95/3 which is totally different from answer that I got using equations in terms of rate.
Where am I going wrong?
why can't we solve this question using equations in terms of time.
please help
Let's think in terms of rate and time to see what's going on. Since we've got one job to do, we'll say

Work = Rate * Time
1 = (m + p) * t
1/(m + p) = t

Notice how this is slightly different from your first rate? Since they're working together, their joint rate is 1/(m + p), NOT 1/m + 1/p.

Now we've got

1/(m + p) = 45

and

1/(m + 2m) = 20

which gives us p = 1/180. This is Perry's rate, so his time is 180 minutes, or 3 hours.

I wouldn't recommend doing it this way, though: the interpretation of the numbers is counterintuitive.
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by Matt@VeritasPrep » Wed Feb 01, 2017 5:28 pm
neha shekhawat wrote: why can't we solve this question using equations in terms of time.
Whoops! I forgot to address this part. We definitely can, as I showed above ... we just might not want to. The rate approach is less prone to error: it's easier to set up and the results make more immediate sense.
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