book Maths Puzzle Numerical Ability

I am truly stumped with this one.

An alloy weighing 20 kg is 30% copper. How many kg of another alloy, which is 80% copper, must be added in order to produce an alloy which is to be 50% copper?

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2) Use the geometric sequence of numbers 1, 3, 9, 27, � to find the following:
a) What is r, the ratio between 2 consecutive terms?
Answer:
Show work in th The decibel level of sound is given by D=10log(I/10^-12)where I is the
sound intensity measured in watts per square meter. Find the
decibel level of a whisper at an intensity of