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You are here: Home » GMAT Test Prep Questions » Linear Equations » Question 6

A children's gift store sells gift certificates in denominations of $3 and $5. The store sold 'm' $3 certificates and 'n' $5 certificates worth $93 on a Saturday afternoon. If 'm' and 'n' are natural numbers, how many different values can 'm' take?

1. 5
2. 7
3. 6
4. 31
5. 18
   

The correct choice is (C). 6 different combinations

1.   Total value of all certificates sold = $93.
2.   Certificates sold were in denominations of $3 and $5.
3.   Both 'm' and 'n' are natural numbers.

The value of all certificates sold, 93 is divisible by 3.

So, a maximum of 31 $3 certificates and no $5 certificates could have been sold.
However, the question states that both 'm' and 'n' are natural numbers.
Hence, at least 1 $5 certificate should have been sold.

If we reduce the number of $3 certificates from the maximum the theoretical maximum of 31 by say 'x' and correspondingly increase $5 certificates by 'y', then 3x = 5y as the value of $3 certificates reduced should be the same as the value of $5 certificates increased.

It means that x has to be a multiple of 5 and y has to be a multiple of 3.

Or $3 certificates reduce in steps of 5 certificates.

So, the following combinations are possible
1.   m = 26, n = 3
2.   m = 21, n = 6
3.   m = 16, n = 9
4.   m = 11, n = 12
5.   m = 6, n = 15
6.   m = 1, n = 18

An alternative way to think of the same concept - Replacing five $3 certificates with three $5 certificated leads to no change in the overall value of certficates sold and gives us a new combination each time. We need to see how many such combinations are possible.