We can answer the problem using a rule of three, simple, direct proportion and also using fractions. First the rule of three:
8 apples ------> 15 min
16 apples ----> x
x = (16)(15)/8
x = 30
therefore he can eat 16 apples in 39 minutes.
We can also equate fractions like so:
8/15 = 16/y
and solve for y:
y = 15*16/8
y = 30
the same answer
