exam Maths Puzzle Numerical Ability

This is a last Subsitution question, but this one says solve by either ;addition or subtraction.

The problem is:
12a-5b=18
15a-5b=30

Please and Thank You=)

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exam Other Question
Stephen purchases a used car for $8000 knowing it will depreciate in value by 10% each year.Calculate the common ratio between successive values for the first four years.Determine Can someone please show me how to do these??

Find theta to four significant digits for 0 < or = to ( less than or equal to) theta < 2pi for:

a) cos theta = -0.9135
b) sin th