
Doug C. answered 11/28/19
Math Tutor with Reputation to make difficult concepts understandable
The system of three linear equations to solve for A, B, C:
100A + 400B + 500C = 2200
7A + 7B + 9C = 65
250A + 1050B + 150C = 4550
There are several techniques available for solving that system. If using addition/elimination then perhaps each equation should be simplified as much as possible. For example, every term of 1st equation is divisible by 100 and every term of of 3rd equation is divisible by 50.
A + 4B + 5C = 22
7A + 7B + 9C = 65
5A + 21B + 3C = 91
Perhaps you know how to use REF or RREF on your graphing calculator?
Or simply choose to for example eliminate "A" from equations 1 &3 and from 1&2, resulting in 2 equations and 2 unknowns.