Question 63777
A sewage treatment plant has two inlet pipes to its settling pond. One (A)can fill the pond in 10 hours,the other(B) in 12 hours. If the first pipe is open for 5 hours and then the second pipe is opened, how long will it take to fill the pond?
A CAN FILL IN 1 HR...........1/10 TANK
A CAN FILL IN 5 HRS ...........5/10=1/2 TANK
REMAINING TANK TO BE FILLED............1-1/2 = 1/2 TANK
B CAN FILL IN 1 HR..........1/12 TANK
A+B CAN FILL IN 1 HR.....(1/10)+(1/12) = 22/120=11/60 TANK
A+B NEED .......60/11 HRS .........TO FILL 1 TANK
A+B NEED .......60/(11*2)=30/11HRS....TO FILL REMAINING 1/2 TANK.
HENCE IT TAKES A FURTHER 30/11 HRS TO FILL THE TANK...OR...
TOTAL 5+30/11 = 85/11=7.73 HRS TO FILL THE TANK