Question

If x+y=8, y+z=7, and x+z=5, what is the value of x? What is the formula or strategy to approach this question?

Answer 1

The strategy is to work from a system with three variables and three equations down to something with two variables and two equations. In a two variable-two equation system, you can use either substitution or elimination.

First, let's organize the equations.

(1) x + y = 8

(2) y + z = 7

(3) x + z = 5

Now, we'll solve one of (1), (2), and (3) for a variable. The choice is yours, but we'll use equation (3) and solve it for x.

x + z = 5

x = 5 - z.

Now we put this into equation (1). Why (1)? It's the only one of (1) and (2) with x.

x + y = 8

5 - z + y = 8

-z + y = 3

y - z - 3 to rearrange it.

Now look at this new equation - call it (4) and the original (2).

(4) y - z = 3

(2) y + z = 7

This is a system of two equations and two variables. Either substitution or elimination works here - let's use elimination because of the -z and +z.

y - z = 3

y + z = 7

We add them together and we have that 2y = 10. Divide on both sides and y = 5. One variable down, two to go.

Now we go back to original equation (2). Substitute y = 5 to find z.

y + z = 7

5 + z = 7

z = 2.

Two down, one to go. Since we know z = 2, let's put it into (3) and find x. (Equation (1) with y = 5 works fine as well.)

x + z = 5

x + 2 = 5

x = 3

Thus x = 3, y = 5 and z = 2.