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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book Other Question
The sound of thunder travels away from a lightning flash at 331m/s. Joe sees a
lightning flash, & then hears the thunder 12 seconds later. Howfar away is Joe from the lightning flash? please help me im doing my homework can you help me on this.

(-2n)(-n^2)