book Maths Puzzle Numerical Ability

i need help solving this equation: r(R1+R2)=R1R2,for R

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book Other Question

How do you solve this problem;

ab divided by c = d.

and

a+a times c = to d

3) Evaluate. 81 ÷ 9 - 16 ÷ 8

i thought was 8 but i was wrong.