What does it mean that something has a 50% chance?
A probability is a number between 0 and 1 (or between 0% and 100%) that says how often something happens if you repeat it many times. A fair coin lands heads with probability 1/2. A die shows a six with probability 1/6. Something impossible has probability 0, something certain has probability 1.
Two rules cover almost everything we need. If two outcomes cannot happen together, the chance of one or the other is the sum: a die shows a five or a six with probability 1/6 + 1/6 = 1/3. If two events do not affect each other, the chance of both and is the product: two coins both land heads with probability 1/2 × 1/2 = 1/4.
You flip a fair coin 10 times. Do you always get exactly 5 heads? The chance of exactly 5 heads in 10 flips is 252 / 1,024, about 25%. Four or six heads are almost as likely. A probability says what happens on average over many tries, not in each one.
The more often you repeat, the closer the share gets to the probability. This is the law of large numbers. The wandering shrinks, but slowly: with 100 flips you typically land within about 5 heads of 50; with 10,000 flips, within about 50 of 5,000, that is half a percent.
Why start a course on quantum computers with coins? Because quantum physics predicts only probabilities. In module 2 each photon landed at a random point; only after thousands of them did the fringes appear. Reading out a quantum computer is like flipping a very special coin, and you need to know how coins behave.
Probabilities of all the possible outcomes add up to 1.
Independent events: P(A and B) = P(A) × P(B). Outcomes that exclude each other: P(A or B) = P(A) + P(B).
Exactly k heads in N flips: P = C(N, k) / 2^N, where C(N, k) counts the ways to choose k flips out of N. For N = 10, k = 5: C(10, 5) = 252, and 252 / 1,024 ≈ 0.246.
The typical spread of the number of heads is √(N p (1 − p)): for N = 100 and p = 1/2, it is 5. Relative to N it shrinks like 1/√N.
A probability tells you how often something happens over many repeats, never what happens in one try: exactly how quantum predictions work.