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Lesson 15 of 16

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Twelve quick questions, one for each lesson. Use the table if you get stuck.

Probability — cheat sheet
IdeaRemember
The ruleP(E) = favourable ÷ total, when all outcomes are equally likely.
Two spinnersWrite each result as a pair, in order. One cell each.
Two dice36 pairs. The 11 sums are not equally likely.
0 to 1Impossible = 0. Sure = 1. Never below 0 or above 1.
Equal chanceCount slips, not the numbers on them.
NotP(not E) = 1 − P(E).
OrCount each outcome once. Check for shared ones first.
All togetherCount all, check equal, count wanted, use the rule, check.
CoinsList every result in order. HT and TH are different.
Pack of 52Count the cards that fit, out of 52.
Number wordsRead each number word. Then list the numbers.
Kept outA marble kept out lowers the total by 1. It lowers its own colour by 1.

A jar has slips 3, 5, 5, 5, 8, 8. You pick one. A student says P(5) = 1/3, "because there are 3 numbers". What is right?

  • 1/2, because 3 of the 6 slips show 5
  • 1/3, because there are 3 different numbers
  • 5/6, because 5 is the number on the slips

Spinner A has parts 1 to 5. Spinner B has parts 1 to 4. How many cells, and what is P(same number)?

  • 9 cells, 1/9
  • 20 cells, 4/20 = 1/5
  • 12 cells, 3/12 = 1/4

Two dice are thrown. What is P(sum 9)?

  • 4/36 = 1/9, because 4 cells make sum 9
  • 1/11, because there are 11 sums
  • 9/36 = 1/4, because the sum is 9

A die shows 1 to 6. A student writes P(7) = −1/5. What is right?

  • 1/5, because the minus sign is a slip
  • 6/5, because 6 faces are counted
  • 0, because no face shows 7

Slips 1 to 8. What is P(more than 2), the quick way?

  • 1 − 6/8 = 2/8 = 1/4
  • 1 − 3/8 = 5/8
  • 1 − 2/8 = 6/8 = 3/4

Slips 1 to 10. A student says P(odd or a multiple of 4) = 7/10. Is that right?

  • No, because two lists always share a slip
  • Yes, because the two lists share no slip
  • No, because the answer should be 3/5

12 cards: 3 colours, each with the numbers 1 to 4. You pick one. What is P(not a 1)?

  • 1 − 4/12 = 8/12 = 2/3
  • 1 − 3/12 = 9/12 = 3/4
  • 1 − 1/12 = 11/12

A jar has slips 2, 4, 6, 8, 10, 12. You pick one. What is P(a multiple of 4)?

  • 3/6 = 1/2
  • 3/12 = 1/4
  • 2/6 = 1/3

Three coins are tossed together. A student counts four head-groups. She says P(at least one tail) = 3/4. What is right?

  • 3/4, because three of the four groups have a tail
  • 1/2, because a coin shows a tail half the time
  • 7/8, because only HHH has no tail

One card is drawn from a pack of 52. What is P(a red card or a queen)?

  • 28/52 = 7/13
  • 30/52 = 15/26
  • 26/52 = 1/2

Tickets are numbered 1 to 20. One is drawn. What is P(a number between 5 and 12)?

  • 8/20 = 2/5
  • 7/20
  • 6/20 = 3/10

A bag has 3 red and 9 green marbles. A green marble is kept out. What is P(the next marble is green)?

  • 9/11
  • 8/11
  • 8/12 = 2/3

Now open Chapter 14, Probability, in your NCERT Maths book. Try its exercises. You are ready.

Reveal every answer to finish this lesson.

That is the end of this subject for Class 10

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