Thomas has two bags of pens.
In the first bag there are 3 blue pens and 7 black pens.
In the second bag there are 2 blue pens and 4 black pens.
Thomas takes at random a pen from the first bag.
He then takes at random a pen from the second bag.
a.[2]
Complete the probability tree diagram.
b.[2]
Work out the probability that Thomas takes two black pens.