9 + 10 does equal 21 and i can prove it.
take a nine and flip it around. what do you get? a 6.
now take the ten and flip it around. what do you get? a 01.
now since we flipped around the parts of the expression, flipping the sum around will give us our answer.
6 + 1= 7
7, when flipped around, is a curved capital L, which is the twelfth letter of the alphabet.
but, since 7 flipped around is a curved capital L, if we curve the 7 and turn it on its side, we get a U
U = the 21st letter of the alphabet, proving 9+10=21