exam Maths Puzzle Numerical Ability

I need to see how to solve the following problem:
-3(8 - c)= 7c

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exam Other Question

Solve each system by the addition method.

(1/8)x+(1/4)y=3
(1/16)x+(1/2)y=7

and

1.5x-2y=-0.25
3x+1.5Y=6.375 If 100 grams of a radioactive material diminish to 80 grams in 2 years, find the half life of the substance