Pipes and cistern Two pipes can fill a cistern in 20 and 34 minutes respectively. Both pipes being opened, find when the first pipe must be turned off so that it filled in 17 minutes?

1 minute A=1/20
1 minute b=1/30
1 minute A+B=1/20+1/30=1/12
Total time to fill is 12 minute
New time 17 minute
Extra time 17-12=5 minute
B can fill tank in 5 minute=1/30*5=1/6
Tank filled is 1-1/6=5/6
Time is taken to fill 5/6 of tank be x
1 min tank filled =1/12
let in x minute tank filled =5/6
cross multiply 1/x=1/12/5/6
x=12*5/6=10 minutes