There are $20$ balls in an urn: $18$ red, $2$ green.
We have to draw $3$ balls and they can be put back (replaceable), so the sample space is $20^3$, or $8000$.
What is the chance of picking exactly 1 green?
What is the chance of picking exactly $2$ green?
I don't understand how to do these.
Edit: For question 1, I tried doing $18\cdot 18\cdot 2.$