a + b + c = 10

a + b - c = 3

find the value of

a + b + 3c

Ok if you look at the first to equations carefully you will see that 2 things have changed between the 2. the "+c" is now "-c" and the "10" is now "3"

if we think about it you will see that c=7. well how do we know. when c or 7 was taken away from the problem it left you with 3

so that means a+b=3

we dont need to know what the exat value of a or b is.

so a+b + 3c

(3) + 3 (7)

3 + 21

24

a+b+3c=24