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

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 
A+B=7, A=7B(1)
A+C=12,(2), frm 1 substitute the value of A into eq 2 :7B+C=12, C=127+B(3)
also B+C=15(4), also in the same way put C in eq3 into eq 4 ;B+127+B=15, B=5, now rem eq1 put B into it ;A=75=2 do the same thin 4 C by putin B in eq3 C=5