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

Lesson 1: Building crowds- Start here!
- Introduction to combinatorics
- 1. Counting with tables
- Table of combinations
- 2. Robot combinations
- Robot combinations
- 3. Tree challenge
- 4. Counting with trees
- Tree of combinations
- 5. Casting challenge
- Casting challenge
- Getting to know Fran Kalal
- Hands-on activity

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# 2. Robot combinations

Let's review the multiplication principle which allows us to quickly count the number of possible robots.

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## Video transcript

- In the previous video and
exercise, we saw how a table is a great way to keep track
of a lot of different kinds of robots, where each robot
is made up of one head and one body. Let's call each of those
different robots a combination. You experience combinations all the time. For instance, when you
wake up in the morning and you pick out a top
and some bottoms to wear, that's a combination. Notice that since each cell
in the table corresponds to a different combination,
we just need to count the number of cells, but we
don't have to count one by one. That's because the number
of cells in a table is just the number of rows
times the number of columns. So, with two heads and three bodies, we have two times three or
six different combinations. And, with three heads and four bodies, we have three times four or
12 different combinations. The next exercise will give you a chance to practice with other combinations.