[caveat emptor]

"Tell how to make 10 when adding 8 + 5."

Silly idea and a waste of time except possibly as an exercise in an abstract algebra course in developing the system of integers, starting with the natural numbers (positive integers + 0) using the Peano axioms, but it can be done. The teacher just screwed it up. Short version.

8+5 =
8+ (2+3) =
(8+2)+3 =
10+3 = 13

[Exit148]

I have 11fingers.— common core math. 1-2-3-4-5———6-7-8-9-10.

On one hand, start with #10 and count down. 10-9-8-7-6. Then add the 5 fingers of the other hand. 6 and 5 are eleven.

So or not so?