Why did we divide by 11.8 and not 18.2 here? Isnt the original one 18.2 . Why is that not the denominator?

Ans is (18.2-11.8)/ 11.8

i.e 6.2/11.8 = 54%

Percentage increased thus , starting value went from 11.8 to 18.2 (exceed the population is the hint)