Calculating Simple Probability · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

Calculating simple probability

Mathematics · Probability · ages 10-11
Name ______________________   Date ____________

  1. In the probability formula, which number goes on the bottom of the fraction?

    • The number of favourable outcomes
    • The total number of possible outcomes
    • The number of unfavourable outcomes

  2. Mia says the probability of rolling an even number on a die is 1/2. Is Mia right?

    Circle one:   True   False

  3. What is the probability of rolling a 4 on a normal six-sided die?

    • 1/4
    • 1/6
    • 4/6
    • 1/2

  4. A bag has 3 blue marbles and 7 red marbles, 10 marbles in total. What is the probability of picking a blue marble?

    • 3/10
    • 3/7
    • 7/10

  5. A bag holds blue marbles and red marbles. Adding more blue marbles increases the probability of drawing blue. True or false?

    Circle one:   True   False

  6. A spinner has 8 equal parts with 3 red. What is the probability of landing on red?

    • 3/5
    • 3/8
    • 1/3
    • 8/3

  7. A bag holds 2 green, 3 yellow and 5 blue balls. What is the probability of drawing blue?

    • 5/3
    • 1/5
    • 1/3
    • 1/2

  8. A spinner has 5 equal sections: 2 are red and 3 are green. What is the probability of landing on red?

    • 2/5
    • 3/5
    • 2/3

  9. A bag has 2 red counters and 8 blue counters, giving a probability of 2/10 for red. If 4 more red counters are added and none are removed, what happens to the probability of picking red?

    • It decreases, because there are now more counters overall
    • It stays the same, because probability never changes
    • It increases, because there are now more favourable outcomes

  10. Zara has a bag with 3 orange counters and 5 purple counters. She says the probability of picking orange is 3/5. What went wrong?

    • She used the wrong number of favourable outcomes
    • She used only the purple counters as the total instead of all the counters
    • Nothing went wrong, she is correct
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Answer key

For grown-ups. Fold this page away before handing over the rest.

Calculating simple probability W1-mt_Zt30Gxi-qp-s1

  1. The total number of possible outcomes · The formula is favourable outcomes ÷ total outcomes, so total outcomes is the bottom number.
  2. True · 3 even faces out of 6 is 3/6, which simplifies to 1/2.
  3. 1/6 · 1 face out of 6 is 1/6.
  4. 3/10 · 3 favourable outcomes (blue) out of 10 total marbles gives 3/10.
  5. True · The bag holds red marbles too, so blue is only part of the whole. The added marbles are all blue, so the blue share climbs.
  6. 3/8 · 3 red parts out of 8 equal parts is 3/8.
  7. 1/2 · 5 blue out of 10 balls is 5/10, which simplifies to 1/2.
  8. 2/5 · 2 favourable (red) sections out of 5 total sections gives 2/5.
  9. It increases, because there are now more favourable outcomes · Adding red counters raises the number of favourable outcomes, so a bigger share of the bag is now red.
  10. She used only the purple counters as the total instead of all the counters · The total should be all 8 counters (3 + 5), so the correct probability is 3/8, not 3/5.
Worksheet · LightMySky