The sum of two numbers is 15. If one number is x, express the other in terms of x.

Answer 1

let x and y be the two numbers - y being the "other" number

since they add up to 15

x + y = 15

express other number "y" in terms of x by solving for y (subtract x from both sides of the equation):

x + -x + y = 15 -x

0 + y = 15 - x (sum of number and its negative is zero)

y = 15 = x (0 is additive identity)