Solve each of the following systems of simultaneous equations using Excel
(a) 15x1 + 20 x2 = 25
5x1 + 10 x2 = 12
(b) 4x1 - 2 x2 + x3 = 6
x1 + 3 x2 - 7x3 = 34
-2x1 - 5 x3 = 14
(c) x1 + x2 + 5 x3 - 12 x4 = 29
x2 + 16 x4 = 21
7 x1 + 2 x2 - 12 x4 = 5
x3 - x4 = 6