[求助]problem solving

yahoo750

On a certain road,10 percent of the motorists who exceed the posted speed limit and receive wpeeding tickets, but 20 percent of the motorests who exceed the posted speed limit do not receive speeding tickets. What percent of the motorists on that road exceed the posted speed? The correct answer is 12.5%. But how to obtain it?

tongxun

let the motorists as x
let the motorists who exceed the posted speed limit as y
(100%-20%)y=10%x, 80%y=10%x, y/x=12.5%

yahoo750

Thank you for your answer!