Three pipes, A, B, & C are attached to a tank. A & B can fill it in 20 & 30 minutes respectively while C can empty it in 15 minutes. If A, B & C are kept open successively for 1 minute each, how soon will the tank be filled? Pick
A) 2 hours  B) 4 hours  C) 3 hours  D) 5 hours  E) 6 hours
in three minute 1/20+1/30-1/15=1/60 part is filled   3 min--------1/60 parts   x min--------- 1 part(full)   x=180 min =3 hours   ANSWER:3 hours
<result>C</result>
Three pipes, A, B, & C are attached to a tank. A & B can fill it in 20 & 30 minutes respectively while C can empty it in 15 minutes. If A, B & C are kept open successively for 1 minute each, how soon will the tank be filled?