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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

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





Practice

Find the Sum of the two Numbers.

  +     =     


You have correct and   incorrect.  This is percent correct.

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