Rachel mows 5 lawns in 8 hours. At this rate how many lawns can she mow in 40 hour?

We can think of this as an equality

5 Lawns = 8 Hours

If she needs 40 hours, you can multiply 8 hours by 5 to get 40, but now it's not balanced. So, you need to multiply 5 to the other side too so:

5 x (5) = 8 x (5)

Therefore she mows 25 Lawns in 40 Hours