exam Maths Puzzle Numerical Ability

This is from the Chapter 4 - test, and I have no notes or examples.

s=35 + 0.5s

just not sure what I do.
Thanks

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exam Other Question

12xy(exp)4/
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8x(exp)2y(exp)3 (exp)=exponent
Use the Substitution method to solve the system of equations.

y - 2x = -5
3y - x = 5