Give one example of when would use the friendly numbers strategy to substract. explain why

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.

C. 40,320 just did the math right now