It's very simple
1m=100cm
4m=4×100=400cm
You could cross cancel the a and your final answer would be 5/11
X - Ollie's age
8 is subtracted from Ollie's age = x-8
the result is the same as
subtracting 16 from 3 times his age = 3x-16

Ollie is 4 years old.