How to mentally add two twodigit 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 twodigit 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 twodigit 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