Step-by-step Answer:
Given:
Bill : 3 minutes / apple
Frank : 5 minutes / 2 apples, or 2.5 minutes / apple.
So in 15 minutes, Bill eats 5 apples, and Frank eats 15/2.5 = 6 apples for a total of 11 apples.
To eat completely at least 12 apples, it would take 15 minutes + 1 apples, which means 17.5 minutes, with Bill having eaten 5 5/6 apples, and Frank having eaten 7 apples, exactly.
So the minimum time to eat 12 apples would be 17.5 minutes.
Note: at 18 minutes, they would have eaten 13 1/5 apples.
