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

Adding 2-digit numbers mentally

There are several strategies that you can use when adding two 2-digit numbers mentally.

Strategy 1:    Break the second addend in parts

45 + 27 = ?   Break the 27 into tens and ones.  First add 20, then add 7: 45 +  27  =  45 + 20 + 7  = 65  + 7  =  72.

1.  Add by breaking the second addend into tens and ones.

52 +  26  = 52 + 20 + 6  = + 6  =

38 +  35  = 38 +      +     = +  =

25 +  57  = 25 +      +     = +  =

62 +  19  = 62 +      +     = +  =

2.  Add mentally.

34 + 35 = 47 + 12 = 33 + 56 = 27 + 26 =

19 + 18 = 13 + 17 = 34 + 18 = 45 + 35 =

 

Strategy 2:    Add the tens and ones separately

45 + 27 = ?   Add tens.  Add ones.  Add the two sums: 45 + 27  =  (40 + 20) +  (5 + 7)  = 60  +   12   =  72.

3.  Add by adding tens and ones separately.

36 + 22  = (30 + 20) +  (6 + 2)  = +     =	72 + 18  = (70 + 10) +  (2 + 8)  = +     =	54 + 37  = (50 + 30) +  (4 + 7)  = +     =
24 + 55  = (__ + __) +  (_ + _)  = +     =	36 + 36  = (__ + __) +  (_ + _)  = +     =	42 + 68  = (__ + __) +  (_ + _)  = +     =

3.  Add mentally, using either strategy - or one of your own!

36 + 38 = 23 + 57 = 27 + 41 = 62 + 35 = 38 + 49 =

22 + 36 = 47 + 34 = 66 + 37 = 27 + 24 = 72 + 19 =

Adding three numbers

The basic strategy is simply break the numbers in parts and add them little by little. You can add all the tens and all the ones separately, or add in whatever parts are easy to add.  You can add the two last numbers first or do whatever works!   33 + 28 + 16  =  (30+20+10)  + 3+8+6  = 60  + 17   = 27 + 39 + 38  =  (20+30+7+9) +  38  = 50 + 16  +  38  = 66 +  30 + 8  = 96 + 8  = 104 You can also add first those ones that would form a ten. 15 + 27 + 45  =  (5 + 5 + 10 + 40)  + 27  = 60  + 27   = 18 + 52 + 16  =  (8 + 2 + 10 + 50) +  16  = 70  +  16  =

4.  Add mentally.

22 + 43 + 38 = 37 + 18 + 15 = 22 + 43 + 38 =   24 + 13 + 25 = 54 + 16 + 22 =

28 + 18 + 36 = 51 + 15 + 16 = 63 + 12 + 17 =   17 + 60 + 20 =   50 + 31 + 19 =

Practice: Addition with mental math