A bag has:
$3$ red
$5$ black
$8$ green
marbles.
Total of 16 marbles.
You select a marble, and then another one right after. (without replacement).
What is the probability that $both$ are red?
Probability that first pick is red: $\frac{3}{16}$
Probability that second pick is red: $\frac{2}{15}$ (since one ball is removed)
Probability of both marbles being red is: $\frac{3}{16} \cdot \frac{2}{14} = \frac{1}{40}$
How do I do this using combinations only?