How to mentally add two two-digit numbers.
- Add 84 and 35.
- First add the two ones' place digits (4 + 5 = 9).
- Next add the two tens' place digits (8 + 3 = 11).
- The sum of the ones' place digits is less than ten so the answer is 119
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- Add 94 and 67.
- First add the two ones' place digits (4 + 7 = 11).
- Next add the two tens' place digits (9 + 6 = 15).
- The sum of the ones' place digits is a two-digit number so
- decrease the ones' place sum by 10 (11 - 10 = 1)
- increase the tens' place sum by 1 (15 + 1 = 16)
- Combine the tens' and ones' place sums to give the answer of 161
Another method to mentally add two two-digit numbers may be easier in some situations.
- Add 84 and 35.
- First add the two tens' place digits (8 + 3 = 11).
- Next add the two ones' place digits (4 + 5 = 9).
- The sum of the ones' place digits is less than ten so the answer is 119.
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- Add 94 and 67.
- First add the two tens' place digits (9 + 6 = 15).
- Next add the two ones' place digits (4 + 7 = 11).
- The sum of the ones' place digits is more than nine so:
- Decrease the ones' place sum by 10 (11 - 10 = 1.)
- Increase the tens' place sum by 1 (15 + 1 = 16).
- Combine the tens' and ones' place sums to give the answer of 161
