Kris V. answered • 09/29/17

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Arturo is correct. But you can also get the answer by limiting the choices for c and d first.

From 5c + 4d = 79 and 0 < c < d

5c + 4c < 79 ⇒ 9c < 79 ⇒ c ≤ 8

5d + 4d > 79 ⇒ 9d > 79 ⇒ d ≥ 9

Make a table for a few integer values of c and d starting at the above limits, you will find the answer.

c 5c d 4d

8 40 9 36

7 35 10 40

6 30 11 44

Since 35 + 44 = 79, the maximum value for c is 7.

As a bonus, you will get the value for d as well.