Lesson 14 of 16
The big picture
Show on a big screenEvery question here is one rule and a careful count. P(E) is the chance of E, the event you ask about.
| Idea | Remember | Example |
|---|---|---|
| The rule | P(E) = favourable ÷ total, when all are equally likely | wheel, 4 of 10 shaded: 2/5 |
| Two spinners | Write pairs in order. One cell each. | 4 × 3 = 12 cells |
| Two dice | 36 pairs. Sums are not equally likely. | P(sum 7) = 6/36 = 1/6 |
| 0 to 1 | Impossible = 0. Sure = 1. Never outside. | P(7 on one die) = 0 |
| Equal chance | Count slips, not numbers. | slips 1, 1, 2, 2, 3, 4: P(1) = 1/3 |
| Not | P(not E) = 1 − P(E) | P(dry) = 1 − 2/5 = 3/5 |
| Or | Count each outcome once. | odd or multiple of 4 on 1 to 12: 9/12 |
| Coins | List every result in order. HT and TH are different. | 2 coins, a head on the big coin: 1/2 |
| Pack of 52 | Count the cards that fit, out of 52. | a heart: 13/52 = 1/4 |
| Number words | Read each number word. Then list the numbers. | prime on 1 to 30: 10/30 = 1/3 |
| Kept out | A marble kept out lowers the total by 1. It lowers its own colour by 1. | 3 R, 5 B, a red out: P(red) = 2/7 |
In the last row, R is red and B is blue. The bag starts with 3 R and 5 B.
Now open Chapter 14, Probability, in your NCERT book. You know every idea in it. Try its exercises.