GMAT 2-13

13. 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 perecent of the motorists on that road exceed the posted speed limit?

answer: 12.5% 10%/80%= 12.5% ???

Please help and explain!! thanks a lot!!

这题与OG上的一题是一样的

说10%的motorists即exceed speed又receive tickets,设motorists为M,那么M*10%就是即exceed speed又receive tickets的motorists

20%的exceed speed的motorists没有receive tickets,那么可以得出80%的exceed speed的motorists receive tickets，设exceed speed的motorists为E，那么E*80%就是即exceed speed又receive tickets的motorists

那么M*10%=E*80% ==>E/M=1/8=12.5%， E是exceed speed的motorists；M是motorists；正好使问题的答案