Question

The sun of two number is 23. Their difference is 9. Write a system of equations to represent the situation. What are the two numbers?

Answer 1

x+y=23
x-y=9
subtract the second from the first and you get 2y=14
Divide 2 from both side so y=7
and since x+7=23, x=16
so the final answer is x=16, y=7

Answer 2

Let one number be x
and second number be y
ATQ 
x + y = 23 ---> 1st equation
similarly, 
x - y = 9 ----> 2nd equation 
( adding y on both sides )
x = 9 + y -----> 3rd equation
Substituting 3rd equation in 1st equation or substituting value of x in 1st equation.
9 + y + y = 23 
( adding like terms on each side)
9 + 2y = 23
( subtracting 9 from both sides)
2y = 23 - 9
2y = 14
( Dividing 2 from both sides )
y = 7

Substituting value of Y in 1st equation
x + 7 = 23
( subtracting 7 from both sides )
x = 16

So, two numbers are 7 & 16 
or 
x = 16  & y = 7

Hope this helps you  :)