What is 9-9 equals?

Answer 1

18

Answer 2

99 equals 99 and only 99. That identity is a fundamentally important requirement of mathematics.

If, for example, 99 = 98 then you could subtract 98 from both sides to show that 1 = 0

And then, given any number x, you could show that

x = x*1 (multiplying by 1 leaves the number unchanged)

= x*0 (because we have just shown that 1 = 0)

= 0 (anything multiplies by 0 is 0).

So that proves that any number, x is equal to 0. That is, there can only be one number, and that is 0.

Since that is clearly not the case, then 99 must be equal to 99 and nothing else!