Question

n is 3 but how did they get there?

Answer 1

12.5 = 100 * (1/2)^n
divide by 100:
0.125 = (0.5)^n
this is equivalent to:
$n= \log_{0.5} 0.125$, which equals 3.
(since 0.5 cubed = 0.125)

Answer 2

The bottom line is saying:. How many times do you have to cut 100 in half to get 12.5 ? ... 100 ==> 50 ==> 25 ==> 12.5 . Three times.