Joanna bought only $0.15 stamps and $0.29 stamps.
How many $0.15 stamps did she buy?
(1) She bought $4.40 worth of stamps.
(2) She bought an equal number of $0.15 stamps
and $0.29 stamps.
Whats the trick in this question
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The trick is the number of stamps she bought must be integers.
Let us assume the numbers of $0.15 and $0.29 stamps she bought are equal to x and y, respectively.
We need to determine x.
Statement 1: 0.15x + 0.29y = 4.40
So, 15x + 29y = 440
--> x = (440 - 29y)/15
Now, x and y must be an integer.
Also x and y cannot be equal to zero as 440 is not a multiple of either 15 or 29.
So, x and y must be positive integers.
As x must be a positive integer, (440 - 29y) must be divisible by 15 and hence by both 5 and 3.
Now, 440 is divisible by 5. So, (440 - 29y) to be divisible by 5, y must be divisible by 5.
So, possible values of y are 5, 10, 15, etc...
For y = 5, (440 - 29y) = (440 - 29*5) = 440 - 145 = 295 --> not divisible by 15
For y = 10, (440 - 29y) = (440 - 29*10) = 440 - 290 = 150 --> divisible by 15
For y = 15, (440 - 29y) = (440 - 29*15) = 440 - 435 = 5 --> not divisible by 15
For y ≥ 20, (440 - 29y) ≤ (440 - 29*20) = 440 - 580 < 0 --> x is negative
So, only possible value of y is 10.
So, x = (440 - 29y)/15 = 150/15 = 10
So, statement 1 is sufficient.
Statement 2: x can have different possible values.
So, statement 2 is not sufficient.
Answer : A
Let us assume the numbers of $0.15 and $0.29 stamps she bought are equal to x and y, respectively.
We need to determine x.
Statement 1: 0.15x + 0.29y = 4.40
So, 15x + 29y = 440
--> x = (440 - 29y)/15
Now, x and y must be an integer.
Also x and y cannot be equal to zero as 440 is not a multiple of either 15 or 29.
So, x and y must be positive integers.
As x must be a positive integer, (440 - 29y) must be divisible by 15 and hence by both 5 and 3.
Now, 440 is divisible by 5. So, (440 - 29y) to be divisible by 5, y must be divisible by 5.
So, possible values of y are 5, 10, 15, etc...
For y = 5, (440 - 29y) = (440 - 29*5) = 440 - 145 = 295 --> not divisible by 15
For y = 10, (440 - 29y) = (440 - 29*10) = 440 - 290 = 150 --> divisible by 15
For y = 15, (440 - 29y) = (440 - 29*15) = 440 - 435 = 5 --> not divisible by 15
For y ≥ 20, (440 - 29y) ≤ (440 - 29*20) = 440 - 580 < 0 --> x is negative
So, only possible value of y is 10.
So, x = (440 - 29y)/15 = 150/15 = 10
So, statement 1 is sufficient.
Statement 2: x can have different possible values.
So, statement 2 is not sufficient.
Answer : A
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This question illustrates a common trap on the GMAT.[email protected] wrote:Joanna bought only $0.15 stamps and $0.29 stamps.
How many $0.15 stamps did she buy?
(1) She bought $4.40 worth of stamps.
(2) She bought an equal number of $0.15 stamps
and $0.29 stamps.
For statement 1, we're able to write the equation 15x + 29y = 440, and in high school we learned that, if we're given 1 equation with 2 variables, we cannot find the value of either variable. However, if we restrict the variables to positive integers within a certain range of values, then there are times when we can find the value of a variable if we're given 1 equation with 2 variables.
This common GMAT trap (along with other common traps) is addressed in the following free videos:
https://www.gmatprepnow.com/module/gmat- ... cy?id=1105
https://www.gmatprepnow.com/module/gmat- ... cy?id=1106
https://www.gmatprepnow.com/module/gmat- ... cy?id=1107
Cheers,
Brent
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how x has different values?
like if she bought same number of stamps so .15x + .29x = 4.40
then .44x = 4.40 and x=10.
let me know if i am missing something.
like if she bought same number of stamps so .15x + .29x = 4.40
then .44x = 4.40 and x=10.
let me know if i am missing something.
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We don't know that $4.40 was spent.arkaprabha wrote:how x has different values?
like if she bought same number of stamps so .15x + .29x = 4.40
then .44x = 4.40 and x=10.
let me know if i am missing something.
So, we have 0.15x + 0.29x = ???
Cheers,
Brent
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A savvy test-taker can determine the correct answer here without actually solving.[email protected] wrote:Joanna bought only $0.15 stamps and $0.29 stamps.
How many $0.15 stamps did she buy?
(1) She bought $4.40 worth of stamps.
(2) She bought an equal number of $0.15 stamps
and $0.29 stamps.
Here are the two statements, translated into math:
Statement 1: 15x + 29y = 440
Statement 2: x=y
Clearly, the two statements combined provide enough information to solve for x and y.
Eliminate E.
The two equations above are very straightforward.
Virtually every test-taker will be able to generate them easily.
This means that the correct answer CANNOT be C.
If the correct answer is C, then almost every test-taker will choose the correct answer, making the problem pointless.
Eliminate C.
Statement 2 is clearly insufficient on its own.
If all we know is that x=y, it's possible that x=y=1 or that x=y=2.
Eliminate B and D.
The correct answer is A.
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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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One last fun fact! These sort of equations - algebraic equations in at least two variables for which we only consider integer solutions - are called Diophantine equations, and they've been entertaining and befuddling recreational mathematicians for about 2000 years.
Here's a nice rundown of the general approach to a simple Diophantine equation: https://mathforum.org/library/drmath/view/51595.html
And here's a fun set of problems (with detailed solutions!): https://www.cemc.uwaterloo.ca/events/mat ... -Solns.pdf
Here's a nice rundown of the general approach to a simple Diophantine equation: https://mathforum.org/library/drmath/view/51595.html
And here's a fun set of problems (with detailed solutions!): https://www.cemc.uwaterloo.ca/events/mat ... -Solns.pdf