Back down through ten to subtract

Answers:

1. What facts do children need to know to use the back down through ten to subtract strategy?

Decomposing numbers less than 10 into parts where one part is nearly as big as the number.
Using the base 10 structure of teen numbers to subtract the ones digit. For example: 14-4=10
Know the facts 10-1 and 10-2

2. Which facts can be efficiently solved using the back down through ten to subtract strategy?

Facts where the minuend is greater than 10 (and no more than 18) and the subtrahend is 1 or 2 more than the ones digit of the minuend.

3. Solve 15-7 using the build up through ten to subtract strategy. Represent your solution process using: