ShmoopTube

00:03

Here's a shmoopy problem for you...

00:05

Mrs. Dolthauser insists on every activity being fairly organized...

00:10

...even a student evacuation when the school is under attack by a mutant sea creature.

00:15

Dolthauser's 20 students are randomly drawing numbers out of a bag to determine

00:19

the order in which they are able to get the heck out of there.

00:22

Each student gets to keep the numbered piece of paper that he or she draws out of the bag.

00:26

Jose picks first and draws a 10, so he will go tenth, behind nine other students.

00:32

Jenna picks next.

00:33

What is the probability that she will pull out a 20...

00:36

and likely be the creature's first victim?

00:38

And here are the potential answers...

00:42

OK, so what is this question really asking?

00:45

Well, it's a slightly more complicated probability question with a twist.

00:49

That is, if it only asked for the odds of pulling one number out of 20 random numbers,

00:54

that'd be pretty straightforward right, but Jose has gone and made things more complicated for us.

01:00

Why? Because he stuck his number in his pocket.

01:02

Because in this case, one number is now being removed -- so the total is no longer 20 -- it's 19.

01:10

So Jenna is drawing for a card labeled "20" out of only 19 possible choices.

01:16

So her odds are now... one in 19.

01:20

With the 10th card removed from the deck,

01:22

her odds of drawing the 20th number just went UP.

01:26

The answer is B.

01:28

Sadly, Jenna does indeed pick the 20.

01:31

Fortunately for her though, the sea creature likes its meat... well done.

Full Transcript