The volume of the water in the pool is (24.5 x 13 x 4) cubic feet.
The number of gallons is (24.5 x 13 x 4 cubic feet) x (7.48 gal/cu-ft)
80% means 0.80 .
To fill the pool to 80% of capacity, it takes
(0.8) x (24.5 x 13 x 4 cubic feet) x (7.48 gal/cu-ft) gallons.