Pipe A can fill a cistern in 36 minutes and B in 48 minutes. If both the pipes are opened together, when should pipe B be closed so that the cistern may be just full in 24 minutes?
Basically you have a rate where for some volume V0, we have dV/dt = a litres/minute where a*36 = V0 and dV/dt = b litres/minute where b*38 = V0 (same volume).
Hint: You need to find a value of t where a*24 + b*t = V0 where t < 24. You have information for a and b in terms of V0 listed above.