Answer
There are ten combinations, namely:
$\text{ab, ac, ad, ae, bc, bd, be, cd, ce, de}$.
$C(5, 2)=10$
Work Step by Step
The combinations are:
$\text{ab, ac, ad, ae, bc, bd, be, cd, ce, de}$.
We know that $C(n,r)=\dfrac{n!}{(n-r)!r!}$..
Hence,
$C(5,3)=\dfrac{5!}{(5-2)!\cdot2!}=\dfrac{5\cdot4\cdot3!}{3!\cdot2\cdot1}=\dfrac{5\cdot4}{2}=10$.
