Question

two pipes can P and Q would feel and MT sister in 24 minutes and 32 minutes respectively both pipes being opened together find when the first 5 mins be turned off so that the empty system maybe just filled in 16 minutes

Answer 1

no thats not the answer. it does not depends on adding or substracting values but depends on the inlet and outlet of the pipes

Answer 2

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