Last reviewed: 11 November 2026
Updated November 2026. These Grade 8 probability notes show how to recognise events that happen by chance, how to carry out a simple chance experiment, and how to write the experimental probability as a fraction, a decimal or a percentage. Probability measures how likely an event is. If you toss a fair coin many times, about half the results should be heads, so the probability of heads is 1/2, 0.5 or 50%.
The examples are original practice for Kenyan junior school learners, not an official paper. They follow the Grade 8 Mathematics probability sub-strand in the KICD Grade Eight curriculum designs. Tree diagrams and later listing of combined events are treated in the Grade 9 probability notes. For Grade 8 topics overall, use the Grade 8 mathematics revision guide and the Grade 8 notes hub.
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Events Involving Chance
An event involves chance when you cannot be sure of the result before it happens. Tossing a coin, rolling a die, or drawing a ball from a bag without looking are chance experiments. Tomorrow’s football score, or whether it will rain on a given afternoon, are also uncertain, though they are harder to model with a bag of balls. OpenStax Introductory Statistics’ probability terminology describes an experiment as a planned activity with a set of possible results.
Some everyday statements are certain (“the sun will rise”), some are impossible (“a coin will show both faces at once”), and many sit in between. Grade 8 work concentrates on experiments you can repeat and record.
Experimental Probability as a Fraction, Decimal or Percentage
Experimental probability of an event = (number of times the event happened) ÷ (number of trials). Write the same result as:
- a fraction, in lowest terms when you can;
- a decimal, by dividing;
- a percentage, by multiplying the decimal by 100.
A short experiment can wander away from the long-run value. Twenty tosses might give 11 heads even if the coin is fair. More trials usually bring the experimental result closer to the expected fraction.
Worked example: a coin
A learner tosses a coin 20 times and records 11 heads. The experimental probability of heads is 11/20 = 0.55 = 55%. The experimental probability of tails is 9/20 = 0.45 = 45%. These two results add to 1, or 100%, because every toss was either heads or tails.
Worked example: a die and a bag of balls
A die is rolled 30 times and a six appears 4 times. The experimental probability of a six is 4/30 = 2/15 ≈ 0.133, or about 13.3%. That is a little below 1/6 (about 16.7%), which is what you would expect from a fair die over many more rolls.
A bag holds 5 red, 3 blue and 2 green balls. One ball is drawn at random. If you only think about equally likely colours as they sit in the bag (before any experiment), the chance of red is 5 out of 10, which is 1/2 = 0.5 = 50%. After 20 draws with replacement, suppose red appears 12 times. The experimental probability is then 12/20 = 3/5 = 0.6 = 60%, close to but not exactly the 50% you first expected.
The fraction simplifier is a handy check when you write 4/30 as 2/15.
Original Practice Questions With Worked Answers
- Which of these is a chance experiment: measuring the length of a desk with a ruler, or picking a card from a shuffled pack? Picking a card, because the result is not known in advance.
- A spinner is spun 25 times and lands on red 10 times. Write the experimental probability of red as a fraction, a decimal and a percentage. 10/25 = 2/5 = 0.4 = 40%.
- In 40 tosses of a coin there are 22 tails. Find the experimental probability of heads. Heads occurred 18 times, so 18/40 = 9/20 = 0.45 = 45%.
- A die is rolled 12 times and every face appears exactly twice. What is the experimental probability of an even number? Even faces are 2, 4 and 6, which appeared 6 times. 6/12 = 1/2 = 0.5 = 50%.
- Why might 20 tosses of a fair coin not give exactly 10 heads? A short experiment can swing either way by chance. The longer you continue, the closer you usually get to one half.
- Express 7/20 as a decimal and as a percentage. 0.35 and 35%.
Cover the answers and try each question. Drawing and reading graphs of recorded results pairs well with the Grade 8 data presentation notes. For timed practice use the Grade 8 CBC Challenge. These notes are free; full revision access is available with the KSh 20 Day Pass or the KSh 500 Monthly Pass.
Frequently Asked Questions
What is experimental probability?
It is the number of times an event happened divided by the number of times you ran the experiment. If red appears on 10 spins out of 25, the experimental probability of red is 10/25 = 2/5.
How do you write a probability as a percentage?
Write it as a decimal, then multiply by 100. 11/20 = 0.55 = 55%. A probability of 1 is 100%, and a probability of 0 is 0%.
Can experimental probability be different from what you expect?
Yes. A fair coin can show 11 heads in 20 tosses. More trials usually pull the result closer to the expected value, such as 1/2 for a fair coin.
What counts as an event involving chance?
Any situation whose result you cannot be sure of in advance, such as tossing a coin, rolling a die or drawing a ball from a bag without looking.
Get the marked answers for the Grade 8 Mathematics paper, KSh 20.
Open the paper