Foundation and Higher · probability basics
Probability.
What you want, over
what could happen.
and how many times
to expect it
when every outcome is equally likely —
P = wanttotal
the outcomes you want
over all the possible outcomes
step 1 of 4
Write the probability
1. 3 of the counters are red.
2. There are 8 counters altogether.
✓ Want over total: 3 over 8
a bag: 3 red counters, 5 blue
step 2 of 4
The scale
1. 0 is impossible. 1 is certain.
2. Every probability sits between them.
✓ 38 is a bit less than evens
step 3 of 4
Probability of not
1. It either happens or it does not.
2. The two probabilities add up to 1.
✓ P(not A) = 1 − P(A)
P(red) = 38
P(not red) = 1 − 38
= 58
step 4 of 4
Expected number
1. Multiply the probability by the trials.
2. 40 ÷ 8 = 5, then 5 × 3 = 15.
✓ An estimate, not a promise
pick and replace, 40 times
expected reds = 38 × 40
= 15
probability —
Expected
probability × number of trials
Write it as a fraction, decimal or %.
Never as a ratio or “3 in 8”.
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