we know that
step 1
find the volume of reservoir
[volume of reservoir]=15*20*10-----> 3000 ft³
step 2
find the volume of water in the reservoir
if it is 56% full
then
volume of water is------> .56*3000----> 1680 ft³
step 3
convert ft³ to gal
if 784 gal------------> 100 ft³
X--------------> 1680 ft³
X=1680*784/100-------> X=13.171.2 gal
the answer is
13171.2 gal