Let

v = 6.4 L/min is the rate of emptying the pool

Since the pool needs to go from 300 L to 60 L, this means that the amount of the water that will go out of the pool is 300 - 60 = 240 L.

t = the time needed until the pool contains 60 L of water only

Since

v = 240/t

we need to solve:

240/t = 6.4

........

click here to see the equation solved for t

........

t = 37.5 min

After 30.5 minutes the pool will have only 60 L of water.