Initially, a large urn contains 3 black marbles and 1 white marble. Every minute, a marble is chosen at random from the urn, and then returned to the urn, together with another marble of the same colour.

If the probability that, after exactly one hour, precisely three-quarters of the marbles in the urn are black can be written as \(\dfrac{a}{b}\), where \(a\) and \(b\) are coprime positive integers, find \(a+b\).

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