Question

"A swimming pool" volume 2400

Answer 1

2400 ft^3 / 8 = 300ft^3 for every foot of depth.
If it is being drained at 30 ft^3 per minute, after 10 minutes 300 ft^3 are drained and the pool depth decreases by 1 foot.
So after 20 minutes, the pool will fall to the 6 foot mark.