We can use the fast algorithm of addition: the formula of addition and subtraction is: both the pre-addend and the post-addend are integers, and the difference between the post-addend and the integer is equal to the sum. So the speed algorithm of 15+8 is 15+ 10-2=23.

Only the formula of the sum of two digits with reversed positions is found: the ten digits of a number plus its single digits multiplied by 1 1 equals the sum. For example, 47+74= 12 1, and the calculation method is (4+7) x11=121.