As we know 1 miles = 5280 feet.
Firstly we will convert everything into feet.
So Length = 28 miles = 28 * 5280 feet = 147,840 feet
Width = 20 miles = 20 * 5280 feet = 105,600 feet.
Depth = 1 inch= 1 * 1/12 feet = 1/12 feet
(because 1 inch = 1/12 feet)
So Volume = Length * Width * Depth
= 147840 * 105600 * 1/12 = 1300,992,000 cubic feet.
So we can say 1300,992,000 cubic feet rain fell.