A deck of 20 cards is randomly shuffled. the probability that the last three cards selected are all odd-numbered cards

combinatoricspermutationsprobability

A deck of 20 cards numbered 1, 2, …, 20 is randomly shuffled. The cards are randomly selected one at a time without replacement, until all 20 have been selected. What is the probability that the last three cards selected are all odd-numbered cards?

My attempt to solve this would be that there are total of 10 odd numbered cards. For card 1 to be chose, it would be 1/17 as the three odd number cards are reserved.

So it would be 1/17 * 1/16 * 1/15….

Is my approach correct?

Best Answer

Hint

By symmetry,
P(last 3 cards are odd numbered) = P(first 3 cards are odd numbered)

You should be able to take it from there !

PS:

I thought this much of a hint was enough, but I see a lot of comments, so for completeness I am giving the answer here. There will be $10$ odd and $10$ even numbers from $1$ through $20$, so

P(last 3 cards are odd numbered) = P(first 3 cards are odd numbered) $= \dfrac{10}{20}\cdot\dfrac9{19}\cdot\dfrac8{18}$