She need to buy 5 cans to paint her house.
Explanation:
Given,
The height of the house(H) = 9 ft
Perimeter =2(L+B)= 140 ft
To find the amount of paint
Formula
Perimeter of a rectangle with L length and B width = 2(L+B) ft
The lateral surface area of a cuboid is = 2(L+B)×H sq ft
Now,
According to the problem,
The lateral surface area of a cuboid is = 140×9 sq ft = 1260 sq ft
Each gallon of paint covers 300 sq ft
Amount of paint is needed =
cans
= 4.2 cans
Hence, she need to buy 5 cans to paint her house.