book Maths Puzzle Numerical Ability

Suppose that one solution contains 50% alcohol and another solution contains 80% alcohol. How many liters of each solution should be mixed to make 10.5 liters of a 70%-alcohol solu

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book Other Question

Express as a single logarithm and, if possible simplify
log(x^3-8)-log(x-2) A collection of dimes and quarters is worth $5.45. There are 32 coins in all. How many of each type of coin are there?

so for this one, i know that we should make D=dimes and Q=quarters.. but thats where my knowledge ends.