What Is Probability and How Do You Calculate It?

La méthode, étape par étape

1. 01

   Define one trial and list everything it can produce

   rolling a fair die: {1, 2, 3, 4, 5, 6} — 6 outcomes

   The complete list is the sample space, and almost every wrong probability comes from listing it carelessly rather than from the arithmetic. Writing it out is worth the time on any question you have not seen before, because the denominator of your answer is sitting in it.

2. 02

   Check whether the outcomes are equally likely

   fair die: yes. A drawing pin landing point-up or point-down: no.

   The counting formula is only valid when every outcome has the same chance. For a biased spinner or a drawing pin the probability has to come from experimental data instead, as relative frequency. Assuming fairness where the question has not stated it is a genuine error, not a technicality.

3. 03

   Count the outcomes that satisfy the event and divide

   P(even) = 3/6 = 1/2

   The numerator counts only the outcomes that make the event happen — here 2, 4 and 6 — and the denominator is every outcome in the list. Keeping them in that order is what makes the answer a fraction of one rather than a ratio, which is a different thing and scores differently.

4. 04

   Place the answer on the 0-to-1 scale and check it

   0 ≤ P(A) ≤ 1 P(not A) = 1 - P(A)

   A probability above 1 or below 0 is not an unlikely answer, it is an impossible one, and noticing that is the fastest error check available. The complement rule follows from the same scale: since something must happen, the chance of an event not happening is whatever is left over.

5. 05

   For two events, decide whether it is "and" or "or"

   and (independent): 1/2 × 1/2 = 1/4 or (mutually exclusive): 1/6 + 1/6 = 1/3

   This single word decides the operation. Two heads on two fair coins is an "and", so the probabilities multiply to 1/4. Rolling a 2 or a 5 on one die is an "or" between outcomes that cannot both happen, so they add to 1/3. Reading the question for that word before calculating is the habit worth building.

01

Everything sits between 0 and 1

A probability is a number from 0 to 1 inclusive, where 0 means the event cannot happen and 1 means it must. It can be written as a fraction, a decimal or a percentage, and all three are acceptable unless the question specifies a form — 1/4, 0.25 and 25% are the same answer.

Because every possible outcome is covered by the list, the probabilities of all the outcomes in one experiment add to exactly 1. That fact is more useful than it looks: it lets you find a missing probability on a spinner or in a table by subtracting the ones you have from 1.

02

The opposite is usually easier

P(not A) = 1 − P(A), and reaching for it is often the difference between one line of working and ten. Questions that ask for "at least one" are the clearest case: the opposite of at least one is none, and none is usually a single short calculation.

For a bag holding 5 red, 3 blue and 2 green counters, P(red) = 5/10 = 1/2, so P(not red) = 1/2 without counting the blues and greens separately. On a larger sample space the saving is far greater.

03

Multiplying, adding, and what changes without replacement

Independent events that both happen have their probabilities multiplied. Mutually exclusive events where either one happens have theirs added. Confusing the two is the most common error in the topic, and the fix is to read the question for the words "and" and "or" before writing anything.

Selection without replacement changes the second probability, because the first item is not put back. From the bag of 10 counters, the chance of drawing two reds in a row without replacement is 5/10 × 4/9 = 20/90 = 2/9 — the second fraction has both a smaller numerator and a smaller denominator, since one red and one counter overall have gone.

A tree diagram is the standard way to keep this straight. Each branch carries its own probability, probabilities along a path multiply, and the probabilities of separate complete paths add.

• "And", independent — multiply: P(A and B) = P(A) × P(B)
• "Or", mutually exclusive — add: P(A or B) = P(A) + P(B)
• Without replacement — reduce both parts of the second fraction
• Along a tree branch multiply; between separate branches add

04

How we teach this

We start every probability question by writing out the sample space, even when a student can see the answer. The habit costs a few seconds on easy questions and is what makes the harder ones tractable, because on those the denominator is the whole difficulty.

Tree diagrams are taught as a way of recording thinking rather than as a topic of their own. A student who understands that a branch means "and" and a fork means "or" does not need to memorise when to multiply.