Balls in a Bag

There are 10 coloured balls in a bag. 7 are red and 3 are blue. You remove one ball and, without replacing the first, then remove a second.

If the second ball is blue, the probability that the first ball was red can be written as \(\frac{a}{b}\) where \(a\) and \(b\) are positive, co-prime integers. Calculate the value of \(a + b\).

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