The perimeter is the sum of the outside dimensions.
b1 = 87 - 32 - 14 - 13 = 28 in.
Area = (32 + 28) /2 x 12 = 60/2 x 12 = 30 x 12 = 360 square inches.
Divide area by the amount 1 can covers:
360 / 50 = 7.2 cans, round up to 8 cans are needed.
Some are said more than once but they is 10 different choices together so it’s be ‘1/10 probability’