From the side, area is that of a rectangle added to a trapezoid or triangle, depending on how you slice it
.. rectangle + trapezoid = (4 ft)*(24 ft) +(4 ft +8 ft)*(16 ft)/2 = 192 ft^2
.. rectangle + triangle = (4 ft)*(40 ft) +(1/2)*(4 ft)*(16 ft) = 192 ft^2
The side area is multiplied by the width to find the volume.
.. V = (192 ft^2)*(20 ft) = 3840 ft^3 . . . . . . maximum water volume
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That's a pint or so more than 28,725 gallons.
