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 20 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) 1 hour 30 min
D) 5 hours
E) 6 hours
in three minute 1/20+1/30-1/20=2/60 = 1/30 part is filled   3 min--------1/30 parts   x min--------- 1 part(full)   x=90 min =1 hour 30 min   ANSWER:1 hour 30 min
<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 20 minutes. If A, B & C are kept open successively for 1 minute each, how soon will the tank be filled?