Mentally Adding Two-Digit Numbers

Mentally Adding Two-Digit Numbers

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
•  
• 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

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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.
•  
• 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