The answer to your question should be 630, if you have a roof the size of 40 feet, by 35 feet.
If the roof is 40 inches, by 35 inches, you would have collected 4.36 inches of rain.
Explanation: To calculate this you are going to do this:
Length of the roof ______ feet * width of roof _______ feet * .6 gallons per square feet *.75 * ________ however many inches of rain
* means multiply
