og question

[waggrave]

123. 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.

I have some trouble dealing with questions like this one. Because with statement 1, I don't know how to determine wether it is sufficient or not. Does someone have a good step by step approach to answer that ?

Thank you

[kmittal82]

DS questions can look simple in the surface, but as always, the devil is in the detail

Let number of $0.15 stamps = x
number of $0.29 stamps = y

We need to find x

(1)
0.15x + 0.29y = 4.40
or
15x + 29y = 440

2 unknowns, 1 equation, looks like its not sufficient, but its always good to test as well. Sometimes, even though you might have 2 unknowns and 1 equation, the solution to that would only be for a specific value of x and y which is apparent from looking at the equation. For the above, one obvious solution is x=10, y=10. Infact, and herein lies the catch, that is the only solution.

So, Sufficient

(2)
0.15x + 0.29x = 4.40
Easy to solve for x

Hence, either statement is sufficient, so (D)

[Brent@GMATPrepNow]

2 unknowns, 1 equation, looks like it's not sufficient, but its always good to test as well. Sometimes, even though you might have 2 unknowns and 1 equation, the solution to that would only be for a specific value of x and y

kmittal82 makes a great point here - many students believe that 2 unknowns and 1 equation automatically makes a statement insufficient.

There are several DS myths like this.

If it helps anyone, (free) videos #12 and #13 at https://www.gmatprepnow.com/module/gmat-data-sufficiency address this DS myth and others.

Cheers,
Brent

[prateek_guy2004]

DS questions can look simple in the surface, but as always, the devil is in the detail

Let number of $0.15 stamps = x
number of $0.29 stamps = y

We need to find x

(1)
0.15x + 0.29y = 4.40
or
15x + 29y = 440

2 unknowns, 1 equation, looks like its not sufficient, but its always good to test as well. Sometimes, even though you might have 2 unknowns and 1 equation, the solution to that would only be for a specific value of x and y which is apparent from looking at the equation. For the above, one obvious solution is x=10, y=10. Infact, and herein lies the catch, that is the only solution.

So, Sufficient

(2)
0.15x + 0.29x = 4.40
Easy to solve for x

Hence, either statement is sufficient, so (D)

Certainly, the ques is asking about 1 value...Thought there are two in the equation..so if we can solve for 1 ...we can get the 2 nd one easily....

similarly in the second statement picking no. are useful.....

Both statements a are sufficient.

Hence D

[saketk]

DS questions can look simple in the surface, but as always, the devil is in the detail

Let number of $0.15 stamps = x
number of $0.29 stamps = y

We need to find x

(1)
0.15x + 0.29y = 4.40
or
15x + 29y = 440

2 unknowns, 1 equation, looks like its not sufficient, but its always good to test as well. Sometimes, even though you might have 2 unknowns and 1 equation, the solution to that would only be for a specific value of x and y which is apparent from looking at the equation. For the above, one obvious solution is x=10, y=10. Infact, and herein lies the catch, that is the only solution.

So, Sufficient

(2)
0.15x + 0.29x = 4.40
Easy to solve for x

Hence, either statement is sufficient, so (D)

Statement 2 is only saying that she bought equal number of stamps.... Can you please tell me how can you find the specific count using this statement?

Answer should be A... Because only Statement 1 is sufficient...

i.e. she bought 10 stamps of each...