With problems such as 86 – 48, some students know how to solve 6 – 8. This leads to  an interesting variation of the standard written forms for subtraction.

Using Number Properties

Problem: “Hoana has $3 in the bank and withdraws $7. How much does she have inthe bank?” Discuss how this can happen. (Answer: Hoana owes the bank $4, so 3 – 7  = –4.)

Hoana uses the method of subtracting negative numbers to work out 73 – 27. She goes 3 – 7 = –4, then 70 – 20 = 50, then 50 – 4 = 46.

So the method correctly shows 73 – 27 = 46.

Examples: Use Hoana’s method to work out:
68 – 49
56 – 28
41 – 23
91 – 78 ...

Problem: “Use Hoana’s method to work out: 423 – 157.”
Examples: Use Hoana’s method to work out: 345 – 189
4 520 – 5 170

Understanding Number Properties:

Explain why Hoana’s method is not useful for every possible subtraction.