Let's find the voume of the pool...it's a "cylinder," so the volume is given by...

pi x r^2 x h = pi x (10 ft)^2 x (4 ft) = about 1256.64 cubic ft

And it is being filled with 20 /7.5 = 8/3 cubic ft of water per min

So...the number of minutes it takes to fill the pool is

1256.64 / (8/3) = 471.24 min

And dividng this by 60, we have...7.854 hrs