a) Choose 2 from 6 to be the first pair that is 6C2 =15
then choose 2 from 4 for the second pair 4C2 = 6
So I think that the answer for part a is 6C2*4C2
$${\left({\frac{{\mathtt{6}}{!}}{{\mathtt{2}}{!}{\mathtt{\,\times\,}}({\mathtt{6}}{\mathtt{\,-\,}}{\mathtt{2}}){!}}}\right)}{\mathtt{\,\times\,}}{\left({\frac{{\mathtt{4}}{!}}{{\mathtt{2}}{!}{\mathtt{\,\times\,}}({\mathtt{4}}{\mathtt{\,-\,}}{\mathtt{2}}){!}}}\right)} = {\mathtt{90}}$$
There are 90 pair combinations.
Now I don't understand the question for part b. Do the 4 new ones partner each other of do they get thrown in before the original 6 get paired off? I'm really confused now. 