Give one example of when you would use the friendly numbers strategy to subtract.

I would say: when we want to substract numbers that are close to each other, but when the respective  digits of the number from which the second number is subtracted are lower than in the other number.

For example, when we want to substract 18 from 21. Those two numbers are very close, but if you did it digit by digit, your would run into a problem.

So instead you can collect their differences to the friendly number 20: 21 is one number away from 20, and 18 two numbers, which makes a total of 3 numbers: and 3 is the solution!

Maybe the answer is 20mm. all i really did was just multiply, which is obvious.