List in order so nothing is missed, decide whether to add or multiply, take out anything counted twice, then use the counts to find a probability.
- 1Systematic Listing7:44
- 2Addition and Multiplication Principles6:39
- Now playing: 3Double Counting3:40
- 4Overlapping Groups6:30
- 5Probability9:40
Counting Principles: Double Counting
Methods in this video
Tap a time to jump to where the video teaches that method.
- 1
Everything counted twice? Halve it
When to use it: Every two people meet once (handshakes, a round-robin tournament, a line between every two points) and you must find how many times in total; or things are weighed or added two at a time, and you must find the total of all of them.
- Add it up person by person, or thing by thing.
- Then ask: how many times is each one in this total?
- If every one is in there exactly twice, halve it. With big numbers this beats listing.
Practise exam-style questions with these methods
Free 7-day trial, no credit card required.