does 9+10 =21

no it doesnt equal 21. It equals 19.

No. My little sister can do that.

This problem constantly gets asked (probably by the same person, presumably a spammer.)

If you don't know how to add:

9: O O O O O O O O O

10: O O O O O O O O O O

Counting the total number of circles, we get 19.

Also you can digit add:

10

+9

19

The one's digit is 9, because 0 + x always equals x.

The ten's digit stays the same, because nothing is added to it.

well done, mathematician... well done