Mental Math within 100

Doing math in your head without pencil and paper.

Using manipulatives, drawing, charts, and writing the numbers by lining them up by place value.

10 + 5 is easier to add because 10 is an easy number to use. To add, we just change the ones place.

A number that is easier to add or subtract with, like 10, 20, or 30. Skip counting means adding the same number over and over, which is also easier.

We can ask “5 plus what number is equal to 9?”

We can subtract one from 6 and add one to 9. Then we have 5 + 10. We can also first add 4 to 6, so that we have 10, and then we can add the remaining 5, so we have 10 + 5 = 15. There is more than one way.

Yes. We can count on from 17: 18, 19, 20. We counted 3 numbers, so 20 – 17 = 3.

We can first subtract 1 from 11 to make 10. Then subtract 3 more so we are subtracting 10 – 3, which is easier. We can also count on from 4 to 11.

The tens place changes but the ones place stays the same.

The tens place changes but the ones place stays the same.