A box contains 5 milk, 8 dark and 3 white chocolates. Jess takes one at random.
- (a)Write down the probability that it is dark.[1]
- (b)Write down the probability that it is not white.[1]
Loading...
Probability measures how likely something is, on a scale from 0 (impossible) to 1 (certain). When outcomes are equally likely, a probability is simply a fraction: the number of ways the event can happen over the total number of outcomes. This lesson builds the foundations every later probability topic depends on.
The big picture
Probabilities are always between 0 and 1 and can be written as fractions, decimals or percentages — never as "1 in 4" or "3 : 1" in an exam answer. The probabilities of all the possible outcomes of an experiment add up to exactly 1.
That sum-to-1 fact is the most useful tool in the topic. It gives , and it lets you find a missing probability in a table — often with algebra.
What you'll learn
An event with probability 0 is impossible; 1 is certain; is an even chance. Words such as "unlikely" and "likely" describe positions on the scale.
For equally likely outcomes: .
A letter is chosen at random from the word PROBABILITY. What is the probability it is a B?
, because an event either happens or it does not.
If all the outcomes of an experiment are listed in a table, their probabilities add up to 1. Use that to find a missing value — including when the probabilities are given in terms of .
The probability of rain tomorrow is 32%. What is the probability, as a decimal, that it does not rain?
Outcomes A, B, C and D have probabilities , , and . Find .
Tip — Check that your final probabilities all lie between 0 and 1 — a negative or bigger-than-1 value signals an error.
Expected number of times an event happens probability number of trials. It is what you would expect on average, not a guarantee.
The probability a seed germinates is 0.85. A gardener plants 240 seeds. How many would she expect to germinate?
Rolling a dice 6 times will often not give exactly one 6. Expected outcomes describe the long-run average — the more trials, the closer the results tend to get. That idea leads directly to relative frequency.
Think like an examiner
Probability rules
Watch out for these
Stretch yourself
A bag contains only red and blue beads. The probability of picking red is . When 10 more red beads are added, the probability of picking red becomes . How many beads were in the bag at the start?
Hint — Let there be beads: red and blue. Form an equation after adding 10 red.
Questions students ask
Key takeaways
Exam-style questions
2 original questions · 6 marks, written to match the style and mark allocation of the real papers. Work on paper, then open the mark scheme and tick the marks you earned — method marks count even if the final answer slips.
A box contains 5 milk, 8 dark and 3 white chocolates. Jess takes one at random.
A biased spinner has four sections. The table shows some probabilities.
Unlimited practice
A new question every time, marked instantly, with a full worked solution. Aim for a streak of five, then move up a level.
Generating a question…
Test yourself
Ready to practise Basic Probability? Pick a mode and earn XP & Dobloons.
Real past-paper questions on Basic Probability, marked mark-by-mark. How you do feeds straight into your weak-topic list, so your revision keeps targeting what actually needs work.