#### Answer

$100-x$

#### Work Step by Step

Let us call the other number $z$.
Since the sum of the two numbers is $100$, we can write the following equation: $z+x=100$
We must solve the equation for $z$, because we are looking for the other number. To do so, we subtract $x$ from each side of the equation: $z=100-x$
Since the other number is equal to $z$, the other number is $100-x$