"Or" means count each one once

A jar has 12 slips, numbered 1 to 12. You pick one without looking. What is P(odd or a multiple of 4)?

The chance is 9 out of 12, which is 3/4.

Check before you add

"A or B" means A, or B, or both. Count each slip once.

• No slip in both lists? Just add.
• A slip in both lists? Adding counts it twice. So list the slips and count again.

Example. A multiple of 3 or a multiple of 4?

1. Multiples of 3: 3, 6, 9, 12.
2. Multiples of 4: 4, 8, 12.
3. 12 is in both lists. So do not just add 4 + 3.
4. The slips are 3, 4, 6, 8, 9, 12. That is 6. P = 6/12 = 1/2.

Now try it. Pick a question. A slip with a ring round it is in both lists.

Try the lab to finish this lesson.