问一道GWD数学题

Q18:

On a certain transatlantic crossing, 20 percent of a ship’s passengers held round-trip tickets and also took their cars abroad the ship.  If 60 percent of the passengers with round-trip tickets did not take their cars abroad the ship, what percent of the ship’s passengers held round-trip tickets?

A.     33 1/3%

B.     40%

C.     50%

D.     60%

E.      66 2/3%

题目说20%的乘客既有round－trip tickets（下面简称r）又take their cars abroad(下面简称ab)

而有r的乘客的60%没有ab，问多少乘客有r

设只有r没有ab的乘客为p％，总人数为x

则(p%x)/(p％x＋20%x)＝60%,所以p％＝30％，

所以有r的乘客的比例为p%＋20％＝30%+20%＝50%

我做的也是50%

设r为round-ticket,p为总人数，则40%r=20%p, 则r/p=50%