Twenty one, the answer

Dude seriously

seriously...

(no comment to this)

$$\mathrm{If\ you \ select\ an\ appropriate\ base\ it\ becomes\ valid:} \textcolor[rgb]{0,0,1}{9_{(ten)}\ +\ 10_{(ten)}\ =\ 21_{(nine)}}$$

$$\textcolor[rgb]{0,0.5,0.5}{Another\ possibility: \mathtt{9_{(twelve)}\ +\ 10_{(twelve)}\ =\ 21_{(ten)}}}$$