Here is one example of when you would use the friendly number strategy to subtract.
42 - 18
We will turn the 18 into a friendly number, which is going to be 20.
(42 + 2) - (18 + 3)
44 - 20
Now it is easier to subtract 44 - 20 than it is 42 -18. With 42 - 18 we would have to barrow whereas with 44 -20 we do not have to barrow and we can do it in our head easier and makes it faster.
