Class 10 Probability Practice: Count Outcomes Without Double-counting

The favourable-over-total shortcut depends on equally likely outcomes. Start by deciding what one outcome means. With two dice, ordered pairs are equally likely; the possible sums are not, because different sums can be made in different numbers of ways.

Curriculum checked 2 October 2026 · original practice for selected skills.

Before you start

You will need: Fractions and systematic counting.

Attempt each task before opening a hint. If you need help, reveal the first hint, then the setup, then compare your written steps with the solution. Finish with the retests without referring back.

Three ideas to keep in view

For a finite equally likely sample space, probability equals favourable outcomes divided by total outcomes.

The complement ‘not A’ has probability 1−P(A). It can be easier to count a single unwanted result than many wanted ones.

For two independent fair dice, there are 36 ordered pairs. (1,2) and (2,1) are different outcomes even though both have sum 3.

Try the worked example first

M10-PROB-W1One fair dieFocused practice

A fair die is rolled. Find the probability of a prime result.

Need a starting hint?

List prime numbers from 1 to 6.

Still stuck? Reveal the setup

They are 2, 3 and 5; 1 is not prime.

Show worked solution
  1. Sample space

    Six faces are equally likely.

  2. Count favourable

    Three prime faces give 3/6.

Answer: 1/2

Before moving on: can you explain why your method works, as well as give the answer?

Your turn: three different checks

Write a method as well as an answer. The tasks change the reasoning, not just the numbers.

M10-PROB-Q1Random drawFocused practice

A bag contains 4 red and 6 blue otherwise identical balls. One is drawn uniformly at random. Find P(red).

Need a starting hint?

Count physical balls, not colour names.

Still stuck? Reveal the setup

There are 10 balls in total.

Show worked solution
  1. Favourable count

    Four balls are red.

  2. Divide

    4/10=2/5.

Answer: 2/5

Before moving on: can you explain why your method works, as well as give the answer?

M10-PROB-Q2ComplementFocused practice

Find the probability of not rolling a 6 on a fair die.

Need a starting hint?

Use 1 minus the probability of 6.

Still stuck? Reveal the setup

P(6)=1/6.

Show worked solution
  1. Complement

    The six has one of six outcomes.

  2. Subtract

    1−1/6=5/6.

Answer: 5/6

Before moving on: can you explain why your method works, as well as give the answer?

M10-PROB-Q3Two diceFocused practice

Two independent fair dice are rolled. Find P(sum is 4).

Need a starting hint?

List ordered pairs giving 4.

Still stuck? Reveal the setup

They are (1,3), (2,2) and (3,1).

Show worked solution
  1. Total outcomes

    There are 6 × 6=36 equally likely pairs.

  2. Favourable outcomes

    Three pairs give 3/36=1/12.

Answer: 1/12

Before moving on: can you explain why your method works, as well as give the answer?

Catch a likely mistake

A tempting claim: “Every named outcome category is equally likely.”

Categories can contain different numbers of equally likely elementary outcomes.

Close the examples and try again

These changed questions test whether you can reconstruct the method. If you use a hint, record where you got stuck and retry later. Completing this small set does not establish full chapter mastery.

M10-PROB-R1Changed bagIndependent retest

A bag has 3 green and 9 yellow identical balls. One is chosen uniformly. Find P(not green).

Need a starting hint?

Use the yellow count or a complement.

Still stuck? Reveal the setup

Nine out of 12 balls are not green.

Show worked solution
  1. Count

    There are 12 balls.

  2. Simplify

    9/12=3/4.

Answer: 3/4

Could you solve this without the earlier example? If not, revisit the first line where you got stuck.

M10-PROB-R2Changed sumIndependent retest

Two independent fair dice are rolled. Find P(sum is 5).

Need a starting hint?

List all ordered pairs.

Still stuck? Reveal the setup

(1,4), (2,3), (3,2), (4,1).

Show worked solution
  1. Count

    Four favourable pairs.

  2. Divide

    4/36=1/9.

Answer: 1/9

Could you solve this without the earlier example? If not, revisit the first line where you got stuck.

Choose the next useful step

If a retest exposed the same error, rewrite the first incorrect step and explain its correction aloud. If both were independent, return to a mixed exercise or a missed paper question. A parent can ask what changed in the method rather than only asking for the answer.

Read the explanation behind this skill

Frequently asked questions

Does experimental frequency always equal theoretical probability?

No. A small experiment can vary. The theoretical answers here follow the stated fair-die and uniform-draw models.

Can I print the questions and worked solutions separately?

Yes. Use the two print buttons above. The question sheet leaves working space and hides solutions; the worked-solutions option includes the solution steps. Your browser can save either view as a PDF.

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Further reading and official sources