Bunuel wrote:

A stationery shop sold pencils for $0.70 each and erasers for $0.50 each. If a girl purchased both pencils and erasers from the shop for a total of $6.30, what is the total number of pencils and erasers the girl purchased? The girl purchased at least one of both the items.

(A) 9

(B) 10

(C) 11

(D) 12

(E) 13

Use integer solutions in two variables to get the answer:

.70P + .50E = 6.30

7P + 5E = 63

We immediately see the first solution since 63 is a multiple of 7.

So P = 9 and E = 0 will be the first solution. Since she buys both pencils and erasers, so this is not our answer.

We know the next solution will be obtained by decreasing P by 5 (co-efficient of E) and increasing E by 7 (co-efficient of P). So next solution is P = 4, E = 7. Total number of items = 4+7 = 11

Answer (C)

Check this post for more on this method:

https://www.veritasprep.com/blog/2011/0 ... -of-thumb/
_________________

Karishma

Veritas Prep GMAT Instructor

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