Relate addition and subtraction facts for numbers to at least 20, eg 5 + 3 = 8, so 8 - 3 = 5 and 8 - 5 = 3

1. 11 + 6 = 17: So what else is true?
2. 11+6=17: So what else is true?
3. 21+5=26: So what else is true?
4. 15+12=27: So what else is true?