Simultaneous equations:
x+3y = 7 ---- (1)
2x+4y = 81 --- (2)
From eqn (1)
x = 7-3y ---- (3)
Substituting for x in eqn (2)
2(7-3y)+4y = 81
14-6y+4y = 81
14-2y = 81
-2y = 81 - 14 = 67
-2y = 67
y = 67/-2 = -33.5
And then, from eqn (3);
x = 7-3y = 7-3(-33.5) = 7-100.5 = -93.5
The solution for the system of equations is;
x = -93.5
y = -33.5
