Nadia is painting the four sides of a rectangular trough. if a can of paint will cover 20 ft2, how many cans will she need to buy to complete the job?

a) 18
b) 21
c) 26
d) 30

The correct answer is B. 21.

Step-by-step explanation:

1. Let's review the information provided to us to answer the question correctly:

Height of the rectangular trough = 8 feet

Length of the long side of the rectangular trough = 20 feet

Length of the short side of the rectangular trough = 6 feet

Square feet a can of paint will cover = 20

2. How many cans will Nadia need to buy to complete the job?

For calculating the answer, we need to know what is the surface of the four sides of the rectangular trough Nadia is painting.

We have two long sides and two short sides.

Surface area of the long sides = Length * Height

Surface area of the long sides = 20 * 8 = 160 feet ²

Surface area of the short sides = Length * Height

Surface area of the long sides = 6 * 8 = 48 feet ²

Now the total surface area of the four sides is:

2 * Surface area of the long side + 2 * Surface area of the short side

2 * 160 + 2 * 48 = 320 + 96 =  416 feet ²

The total surface area of the four sides is 416 feet ²

Finally, to know how many cans will Nadia need, we make this division:

416/20 = 20.8 (Total surface area of the four sides/Surface area a can of paint covers)

But we only can buy whole number of cans, in this case, we round to the next whole number and the answer is B. 21.