Question

What is the value of x in the equation x – y = 30, when y = 15

Answer 1

x - y = 30

Plug in y = 15

x - 15 = 30

Add 15 to both sides

x = 30 + 15

x = 45

That's your answer :D

Answer 2

y = 15
x - y = 30

We replace y with the given value:

x - 15 = 30
x = 30 + 15
x = 45