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### Course: Pixar in a Box > Unit 13

Lesson 2: Counting crowds- Start here!
- 1. Two headed robots
- Counting two-headed robots
- 2. Snake bots
- Building snake bots
- 3. Calculating factorials
- Calculating factorials
- 4. Casting problem
- Counting casts 1
- 5. Does order matter?
- Counting casts 2
- 6. Binomial coefficient
- Combinations

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# 2. Snake bots

Let's build some snakes to get us thinking about permutations. Learn how to assemble different varieties of robots using a set number of segments, where each segment is used exactly once.

## Want to join the conversation?

- At0:15what did they mean by formula coming soon?(7 votes)
- The math formula is shown in a later formula.(1 vote)

- using this same idea you should be able to figure out how many different algorithms there are to solve this problem. anybody have an idea how you could do that?(2 votes)
- It would be quite easy if you are in high school

you just have to calculate the factorial(n!) of the number which means that you have to multiply the number to all the natural numbers less than it.

ex. 5! would be 5 * 4 * 3 * 2 * 1 or 120

6! would be 6 * 5 * 4 * 3 * 2 * 1 or 720(1 vote)

- Why can't they have age friendly math! they have 8th grade math!(1 vote)
- they say that this is more advanced math, but moving at a slower pace with someone who knows more about the topic than you might help you follow along!(1 vote)

- ?formula hmmmm....... oh its so the bot can move around and talk i think heres link/link to formula .//./././/..com(1 vote)
- cause its copyright anywho i did some resherch on that the bot and its not real go see if u want or go to this link /...com@snake bot..com@KHANACDMEY/snkae bot/spider bot also.com@if u go to you get free robux(1 vote)
- did anyone notice the pixar thing?(0 votes)
- at 16 secs they said (chimes ringing) what are they talking about?(0 votes)
- I do not get that thing?(0 votes)
- Try thinking of Slither.io, where every circle is a segment of the snake.

Hope that helps!(1 vote)

## Video transcript

- In the rest of this
lesson, we're gonna look at a few other counting
problems that come up when you try to make a large crowd of robots from a small bin of parts. We're gonna be building up
to a really powerful formula that you can think of as a counting tool. (chimes ringing) To start, let's imagine snake-like robots. Let's call them Snake Bots. They are built as a sequence of segments where every segment must
be used exactly once. For instance, if I'm building
a three-segment Snake Bot, I might assemble the segments this way. Or I could do it this way. We'd again like to know: How many of these Snake Bots can I build? Remember, you have to account
for every possibility. In the next exercise,
you're gonna be asked to build all possible Snake
Bots of length one through four. Go get 'em!