9+10=21

Yes, my calculation says , it is possible.

But only in other number base.

Let me explain.

9 + 10 = 21

or, 19 = 21

Assume that , 19 is a decimal number.

and 21 is a n base number.

As 19 is equivalent to 21,

So 10 base 19 is equivalent to n base 21.

So with the help of math I can get that,

19 = 1 × 10^1 + 9× 10^0

21 = 2 ×n^1 + 1×n^0 = 2n + 1

So , 2n + 1 = 19

or, 2n = 18

or, n = 9

So finally, we can say that,

10 base 19 = 9 base 21

Thus 9+10=21 is possible