## Thursday, September 12, 2013

### Strategies to add or subtract numbers

Today we discussed easy ways to add or subtract numbers mentally (in our heads without pencil or paper or even a calculator). We discussed adding nine to numbers by adding ten then taking away one. Our challenge was to use a similar strategy to record other numbers that might be added or subtracted this way.

Lauren: To add 8 to any number you can add 10 then take away 2.
eg: 64 + 8 = 64 + 10 - 2 = 72

Jacob: To add 23 to any number you can add 20 and then add 3.
eg: 1 648 + 23 = 1 648 + 20 + 3 = 1 671

Tia: To add 13 to any number you can  add 10 and then add 3.
eg: 674 + 13 = 674 + 10 + 3 = 687

Tessa: To take away 9 from any number you take away 10 and then add 1.
eg: 52 - 9 = 52 - 10 + 1 = 43

Tom: To take away 18 from any number you can take away 20 and then add 2.
eg: 147 - 18 = 147 - 20 + 2 = 129

Stephanie: To take 8 away from any number you can take away 10 and add 2.
eg: 63 - 8 = 63 - 10 + 2 = 55

Can you share any other numbers that might be added or subtracted in this way?