Answer:
b=45^1/2
Step-by-step explanation:

In this question, you are given the width( 20 miles), the length(28 miles) and the height( 1 inch). You are asked to count the rain water volume which was going with formula length x width x height. But the unit asked is ft, so you need to convert them all. The calculation would be:
Water volume= length x width x height
Water volume= 20 miles x 5280ft/miles x 28 miles x 5280ft/miles x 1inch x 1ft/12inch
Water volume = 1300992000 cubic ft.
I think it is $30 since it has the highest probability.