Bobby is 3 years younger than 5 times his grandson's age. The sum of their ages is greater than 75. What is the youngest age Bobby's grandson can be?

If his age was 13 then it would add up to exactly 75, but to be greater it would be 14. I would put a note by the question if I were you.

The youngest age Bobby's grandson can be is 16.

Step-by-step explanation:

1. Use variables

Bobby=B Grandson=G

B=5 (G) - 3

B+G is greater than 75

2. Use trial and error

If G is 15 ... 5 (15) - 3=72. This does not work

If G is 16 ... 5 (16) - 3=77 This works

So his grandson must be at least 16 years old.