"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)?
Step 1 of 4
- 12 slips, 1 to 12.
- Odd: 1, 3, 5, 7, 9, 11. That is 6 slips.
- Multiples of 4: 4, 8, 12. That is 3 slips.
- No slip is in both. So 6 + 3 = 9. P = 9/12 = 3/4.
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?
- Multiples of 3: 3, 6, 9, 12.
- Multiples of 4: 4, 8, 12.
- 12 is in both lists. So do not just add 4 + 3.
- The slips are 3, 4, 6, 8, 9, 12. That is 6. P = 6/12 = 1/2.
Watch out!
Adding 4 + 3 = 7 to write 7/12 counts slip 12 twice. Look for shared slips first.
Now try it. Pick a question. A slip with a ring round it is in both lists.
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