As long as the slope of the bottom is constant and continuous, with no flats
or sudden drop-offs, we can treat the whole pool as if its depth is the average
of 3-ft and 8-ft = 5.5-ft.
Volume = 20 x 40 x 5.5 = 4,400 cubic feet = 32,914.3 gallons (rounded)
Note that this is the volume of the pool, not necessarily the amount of water in it.
I think you would also need the distance where the slope up to the shallow end starts.
10,408 gallons.
7,646.4 gallons are needed.
About 24 gallons.
About 18,402 gallons.
Approximately 1,616 US gallons.
1.5x4x6x7.48 gallons/cubic foot=269.28 us gallons
without knowing the actual lengths of the shallow and deep ends, nothing more than a very loose ballpark number is possible.
A 15-foot x 42-foot oval pool with an 8-foot deep end and a 3-foot shallow end will contain about 20,443 gallons of water.
13,594 US gallons of water.
If rectangular about 21,000 gallons if oval about 16,000 gallons
25000 gallons