We are told that 10% who speed get ticketed , so 90 % speed and dont get ticketed, right ?
We are told that 10% of motorists speed and get ticketed. We don't know anything about the other 90% at this point. We can't assume the other 90% speed and just don't get caught. Then they go on to say that 20% of those who speed don't get ticketed.
So 10% of motorists = 80% of speeders. (100% - 20% that don't speed and don't get caught)
to get 100% of speeders (which is what the question is asking) multiply both sides by 1.25.
10% x 1.25 = 12.5% and 80% x 1.25 = 100%
12.5% of motorists speed. B.