On a certain road, 10 percent of the motorists exceed the posted speed limit and receive speeding tickets, but 20 percent of the motorists who exceed the posted speed limit do not receive speeding tickets. What percent of the motorists on that road exceed the posted speed limit ?
(A) 10 1/2% (B)12 1/2%
(C) 15% (D) 22%
Let the no. of motorists on the road be 100x.
No of ppl who overspeed and receive tickets = 10% of 100x = 10x.
The stem says that 20% of those who oversped got away without tickets, hence 80% of the ppl who overspead received tickets.
ie. 80% ( no of ppl who overspead ) = 10x
ie (no of ppl who overspead = 10x/.8 = 12.5x.
which is 12.5 % of the total. Hence B.