Question

two pipes X and Y can seperately fill a tank in 18 and 24 hours respectively. If they are turned on alternately for one hour each, how long will it take to fill the cistern?
a) 20 hrs 30 mins
b) 24 hrs
c) 18 hrs 30 mins
d) 16 hrs​

Answer 1

Answer:

X filling rate is 1/18 or 4/72 per hour

Y filling rate is 1/24 or 3/72 per hour

Combined as a pair it’s 7/72 in a complete 2 hour alternating sequence

72+ 7/72 =10 sequences = 70/72 on 20 hours, The outstanding balance is just 2/72

So at 20.5 hours with X turn filling is 100% complete with 72/72

Answer : It will take 20.5 hours to completely fill this tank.