This is a ratio...
45 out of 125 did not approve. That is written
45:125 or 45/125
you want to find how many out of 25 which would be written
x:25 or x/25
now we can compare
45/125 = x/25
if we divide 125 by 5 we get 25 so we need to divide 45 and 125 by 5 to get the answer.
45/5 = 9
125/5 = 25
Your new ratio is
9:25 or 9/25
9 of 25 people don't agree.