A + b = 15.6
a + c = 18.3
b + c = 13.9
a + b +c =?

Step-by-step explanation:

Before proceeding to solve a system of linear equations, we have to answer the following question:

The number of unknowns equals the number of equations? If this is true, then we will be able to solve the problem

As you can see we have three unknowns:

And we have three equations:

We can solve this system of linear equations in many ways:

Let's use substitution:

Let:

For (1)

Replace (4) into (3)

Replace (5) into (2)

We have found a, so it will be much easier from here:

Replace (6) into (4)

Replace (6) into (5)

Therefore: