Probability · Ordinary Level
Counting — AND & OR
Ordinary Level · Class 2 · Tap NEXT to begin
Section 1 of 2
The AND Rule — Multiply
AND means multiply
When you do one thing AND another, multiply the number of ways.
Worked example — tea and a game
I can get tea in $3$ ways (Home, Town, Canteen) and go to a game in $2$ ways (drive, walk). How many ways can I get tea and go to the game?
List them: Home&drive, Home&walk, Town&drive, Town&walk, Canteen&drive, Canteen&walk
$3 \times 2 = 6$
$6$ ways
Section 2 of 2
The OR Rule — Add
OR means add
When you do one thing OR another, add the number of ways.
Remember
AND = multiply. OR = add.
You try
There are $3$ ways to go for tea and $2$ ways to go to a game. How many ways of going for tea or the game?
It’s an OR question, so add.
Tea: Town, Home, Canteen; Game: walk, drive
$3 + 2 = 5$
$5$ ways
That’s Class 2.
The AND rule (multiply) and the OR rule (add), with sample spaces and tree diagrams. Next: the fundamental counting principle and arrangements.