What's the fastest way to do these math problems?

jamesford
Sat, 26 Sep 2009 18:49:08 +0000

Of the 240 campers at a summer camp, 5/6 could swim. If 1/3 of the campers took climbing lessons, what was the least possible number of campers taking climbing lessons who could swim?

20
40
80
120
200

From the info in the problem, we know 200 can swim and 80 took climbing lessons. At this point, I just tried the choices. 20 who did both would mean 60 who climbed only and 180 who swam only, giving 20 + 60 + 180 > 240, so that’s wrong. 40 climbing and swimming would mean 40 climb only and 160 swim only, which works and must be the answer.

What’s a direct way of obtaining the answer as 40, say if you didn’t have it as an answer choice?