Here is an interesting question that combines linear equations and properties of numbers to get an answer.

Question

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?

A. 5

B. 7

C. 6

D. 31

E. 18

Correct Answer : Choice C. 6 different values

Explanatory Answer

Hence, at least 1 $5 certificate should have been sold.

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.

Question

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?

A. 5

B. 7

C. 6

D. 31

E. 18

Correct Answer : Choice C. 6 different values

Explanatory Answer

Key data :

1. Total
value of all certificates sold = $93.

2. Certificates sold were in denominations of $3 and $5.

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.So, a maximum of 31 $3 certificates and no $5 certificates could have been sold.

Hence, at least 1 $5 certificate should have been sold.

If
we reduce the number of $3 certificates from the maximum 31 that is possible 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

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

Think
about it another way. 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 every time.
We need to see how many times this can be done.

Labels: GMAT Linear Equations, GMAT Number Properties, GMAT Number Theory, GMAT Problem Solving