The volume of a prism is given by v=bh where b is the base and h is the height. In this case, the base is 25 square feet and the height is 3 ft so 25*3=75 cubic feet.
since a year has 12 months, then 3 years will be 36 months, now, 3 years and 9 months is just 36 + 9 = 45 months. So we can say that 3 years and 9 months is really 45/12 of a year.