There are 6 blue pens and k k\,k black pens in a box.
Liam takes a pen from the box at random.
He puts the pen back into the box.
Liam then takes another pen at random from the box.
The probability that both pens Liam takes are blue is 964\displaystyle \frac{9}{64}649
Work out how many black pens are in the box.
