First we need to find the area of roof we need to cover:
Length * Width = Area
23 * 15 = 345
Now we know that we must cover 345 square feet of roof.
Then, we must divide the area of roof by the area each can will cover:
345 / 100 = 3.45
Since it would likely be impossible to buy 45% of a can, we must round up since at the same time we can't leave part of the roof uncovered.
3.45 rounded up is 4.
So, you need 4 cans of sealant.
