F.1 maths

In 1 min, pipe A can fill 1/50 of the water pool.
In 1 min, pipe B can pump away 1/(60 + 30) = 1/90 of the water in the waterpool.

When both pipes are used, fraction of the water pool filled in 1 min
= (1/50) - (1/90)
= (9/450) - (5/450)
= 4/450
= 2/225

When both pipes are used, time taken to filled the water pool
= 1/(2/225) min
= 225/2 min
= 112.5 min

Time that it will be delayed
= (112.5 - 50) min
= 62.5 min