Question

Wo pipes a and b can fill a tank with water in 30 minutes and 45 minutes respectively. the water pipe c can empty the tank in 36 minutes. first a and b are opened. after 12 minutes c is opened. total time (in minutes) in which the tank will be filled up is:

Answer 1

total work=1 part
Work done by pipe a in 30 mins=1 part
work done by pipe a in 1 min = 1/30 part

Work done by pipe b in 45 mins = 1 part
Work done by pipe b in 1 min = 1/45 part

Work done by pipe c in 36 mins =1 part
work done by pipe c in 1 min = 1/36 part

THERFORE,
work done by a + b in 1 min = (1/30 + 1/40) part
=7/120 part
work done by a + b in 12 mins = 7/120*12
=7/10 part
THERFORE, work left = 1-7/10 part
= 3/10 part

work done by a+b+c in 1 mins = (1/30+1/45)-1/36 part=find out yourself.
then that much work =1min
therfore
time required for 1 work=
so, time required for 3/10work = x time
therfore,
total time =12mins + x mins