Question

a + b = 6, b + c = 20, and c + a = 7.5, solve for a , b and c.

Answer 1

3+3=6
10+10=20
4.3+3.3

Answer 2

equation #2 - equation #3:
b - a = 18.5
add this to equation #1
2b = 18.5
b = 9.25

put this into #1
a + 9.25 = 6
a = -3.25

put this into #3
c - 3.25 = 7.5
c = 10.75

Answer 3

One method:
Use substitution to express an equation in one unknown.
Choose a.
From the first equation...
a + b = 6
So, b = 6 - a

From the third equation...
c + a = 7.5
So, c = 7.5 - a

Now write the second equation substituting the values for b and c in terms of a:
b + c = 20
From the above values for b and c in terms of a:
(6 - a) + (7.5 - a) = 20
6 - a + 7.5 - a = 20
13.5 - 2a = 20
-2a = 6.5
Solve for a.
Substitute that to find b and c.