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Marbles

Permanent link to this puzzle: mscroggs.co.uk/puzzles/95
A bag contains $$m$$ blue and $$n$$ yellow marbles. One marble is selected at random from the bag and its colour is noted. It is then returned to the bag along with $$k$$ other marbles of the same colour. A second marble is now selected at random from the bag. What is the probability that the second marble is blue?
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