Question 953413
Two pipes P and Q fill an empty cistern in 24 minutes & 32 minutes respectively.
 Both the pipes being opened together.
 Find when the first pipe must be turned off so that the empty cistern may be just filled in 16 minutes. 
:
let t = on time of the 1st pipe to accomplish this
let the completed job = 1 (a full cistern)
:
each will do a fraction of the filling, the two fractions add up to 1
{{{t/24}}} + {{{16/32}}} = 1
reduce the 2nd fraction
{{{t/24}}} + {{{1/2}}} = 1
multiply equation by 24, cancel the denominators
t + 12 = 24
t = 24 - 12
t = 12 min, then turn the 1st pipe off