Originally posted by SPMars
First hose has a rate of 5/12 litres per second and the other hose has a rate of 5/25 litres per second.
So the combined rate is 5/12+5/25 litres per second.
There 150*1000 litres to fill, so the total time needed is
150*1000/(5/12+5/25) = 9000000/37 seconds.
If you divide out each contibutor, you get 5/12 of a liter per second or 0.41666 L/sec. The other one comes out at 5/25 or 0.2 L/sec. That makes 0.61666 L/sec filling the pool. Inverting that gives 1.6216 seconds per liter, times 1000 gives 1621.6 seconds to fill one cubic meter. Times 150 gives 243,245 seconds to fill the 150 M^3 pool.
The same as SP, but made the final division.
That problem can be restated as the current flowing in parallel in two resistors where they are charging a battery to X amount of joules of energy. It could be stated as 5/12 amp in one and 5/25 amp in the other. Same kind of problem.