Math help? Trade specific?

Question

A typist uses a sheet of paper nine inches by twelve inches. She leaves an inch margin on each side and a one and a half inch margin on the top and bottom. What fractional part of the page is used for typing? The answer is 7/12 but I don't know how to figure this out.

Answer 1

The total area of the paper is 9 x12 =108 square inches

The area for typing is

12 -1.5 -1.5 = 9, by 9-1-1=7  ; 9x7= 63 square inches

63/108 = 7/12  (common factor is 9)