Question 34293
When doing surface area find all the possible combination of the numbers and multiply it by two.

(3*4)+(3*7)+(7*4)=61*2=122 meters^2

This is your surface area for the room.

122/40=3 and a little so you need 4 but you need two coats so you need 7. The decimal when you originally divide is so negligible that you won't need 8 cans.

This tells you how many cans of paint she needs to buy.

7*4=28 liters of paint. 7 equals the number of cans to buy 4 equals the amount in each can.