arithmetic - a water tank can be filled by two pies p1 and p2 working simultaneously in 24 and 32 min respectively both pipes being opened find when the first pipe must be turned off so that the tanker may be just filled in 20 min - skillgun

Solution: Let X be turned off after x min. then,
Part filled by (X+Y) in x min. + part filled by Y in (20 - x) min. = 1.
.'. x(1/24 + 1/32) + (20 -x)1/32 = 1
=> x/24 + x/32 + 20/32 - x/32 = 1
=> 4x+60 = 96
=> 4x = 36
=> x = 9.