"a ∝ b" means "a is proportional to b", which in turn means there is some constant k such that
a = kb
We're given that a = 18 and b = 3, so that
18 = 3k ⇒ k = 6
Then when b = 5, we would have
a = 6 × 5 = 30
The answer is to that is 100
25times 4
Second answer if it still helps
