haichao 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.

I thought it would be worth noting why statement one is enough information since it might not be obvious that the solution of ten 15-cent stamps and ten 29-cent stamps is the ONLY solution here.

Let's use cents rather than dollars.

The most important thing to notice here is that no matter how many 15-cent stamps I buy, their total value will always end in 5 or 0 (e.g., 15 cents, 30 cents, 45 cents, 60 cents etc). So, if we are to combine a certain number of 29-cent stamps, we need enough of them so that their total value ends in either 5 or 0 since all of the stamps are worth a nice tidy sum of 440 cents.

Under what circumstances will the total value of our 29-cent stamps end in either 5 or 0? When we have 0 stamps, 5 stamps, 10 stamps or 15 stamps. We need to check all four of these cases, since it might be possible that there are two or more ways to get 440 cents worth of stamps, in which case (1) would not be enough informaiton.

If we check four cases, only one case (ten 29-cent stamps combined with ten 10-cent stamps) will give us a sum of 440 cents.

This is why statement (1) provides enough information.

Statement (2) does not provide enough information.

The answer is A.