Laurence is painting the walls and ceiling of a rectangular bedroom that is 3 m by 4.5 m. The walls are 2.5 m high.
There is a window measuring 1 m by 0.8 m on one wall and a door that is 1 m by 2.3 m in another wall.
If a can of paint covers 20 m2 and costs $38.95, what will it cost Laurence for paint for this bedroom?

This problem has a lot of steps to it, but the math is not difficult - just addition, subtraction, and multiplication. You're adding up the areas of a bunch of rectangles, so you're basically doing a bunch of different (length x width) calculations.

The room has four walls. The wider walls are each 4.5m wide and 2.5m tall. The narrower walls are 3m wide and 2.5m tall. Add up the area of all four walls, then subtract the areas that don't need to be painted (i.e. subtract the area of the door and the window from the total). That's how much total area needs to be painted.

Since one can of paint covers 20m^2, divide the total area by 20 to figure out how many cans of paint it will take. Finally, multiply the number of cans by the cost per can to figure the total cost.