A box contains 15 counters. There are 4 red counters, 5 green counters and the rest are yellow counters. Niklas takes at random a counter from the box and writes down the colour of his counter. He then puts the counter back into the box. Sasha then takes at random a counter from the box and writes down the colour of her counter. Work out the probability that the counters taken by Niklas and Sasha both have the same colour.

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