The sum of two numbers is 18 and the difference is 4

Let x represent the first number
Let y represent the second number

x + y = 18
 x - y =  4

Use elimination method to solve this system of equations:
2x = 22
 x = 11

Plug the x-value into one of the original equations to solve for y:
x - y = 4
(11) - y = 4
y = 7

The two numbers are 7 and 11

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Cdoes not have two variables