Data Sufficiency

Hi Saurabh,
say x is the nos of motorists on the road
and let y be nos of motorists who exceed the speed limit

so .2y = nos of the motorists who exceed the posted speed limit do not receive speeding tickets

so .8y = nos of the motorists who exceed the posted speed limit & receive speeding tickets

now we have .8y = .1x (i.e. 10 percent of the total motorists exceed the posted speed limit and receive speeding tickets)

so y/x =1/8

% =12.5 percent

Hope this helps u.

samir's explanation is pretty good and straight. hope this clears your doubt saurabh. If not, let me know I will explain my method.