GMAT OG 13

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132. 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.

Can someone explain to me why is statement 2 insufficient? Cant it be written as 0.15(x) + .29(y) = 4.40
since, x=y so we replace y=x in the equation above. Please explain?

[srcc25anu]

It just says she bought an equal number of 15c and 29c stamps but does not tell us the total amount she spend or the # of each type of stamps she bought.
It could be she bought 1 of each kind and then # of 15c stamps bought will be 1
BUT if she bought 10 of each kind then # of 15c stamps bought will be 10
hence B is insufficient

[Brent@GMATPrepNow]

132. 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.

Can someone explain to me why is statement 2 insufficient? Cant it be written as 0.15(x) + .29(y) = 4.40
since, x=y so we replace y=x in the equation above. Please explain?

In statement 2, all we know is that Joanna bought an equal number of $0.15 stamps and $0.29 stamps.
We don't know that she spent $4.40 (that info is in statement 1)

Cheers,
Brent

[ceilidh.erickson]

Be very careful to evaluate each statement individually! You were using what we called "statement carryover" - you were using the information from statement 1 in evaluating statement 2. This is a very easy mistake to make, but it will really hurt your score! You have to "forget" what you knew from the previous statement. I find that it's helpful to actually hold your hand over the first statement so you can only see the second one.

Focus on trying to PROVE INSUFFICIENCY! With the second statement, here are some possible scenarios:

one of each stamp: $0.15 + $0.29
100 of each: $15 + $29
10,000 of each: $1500 + $2900

As we can see, there's no way to tell how many $0.15 stamps we have.