Question

Solve the following system of equations for the unknown variables.
a + b = 7
a + c = 12
b + c = 15

Answer 1

a + b = 7
a + c = 12
----------------subtract
b - c = - 5
then
b - c = -5
b + c = 15
----------- add
2 b = 10
b = 5
you take it from there

Answer 2

A+B=7, A=7-B---(1)
A+C=12,---(2), frm 1 substitute the value of A into eq 2 :7-B+C=12, C=12-7+B---(3)
also B+C=15---(4), also in the same way put C in eq3 into eq 4 ;B+12-7+B=15, B=5, now rem eq1 put B into it ;A=7-5=2 do the same thin 4 C by putin B in eq3 C=5