Foundation and Higher · adding to 1
All the probabilities
add up to 1.
Use it.
the missing one is
whatever is left
outcomes that cannot both happen —
Mutually exclusive
a spinner cannot land on red and blue
so their probabilities add
step 1 of 4
Find the missing one
1. Add up the ones you know: 0.7.
2. Subtract from 1.
✓ P(yellow) = 0.3
0.2 + 0.35 + 0.15 = 0.7
P(yellow) = 1 − 0.7 = 0.3
red · blue · green · yellow
step 2 of 4
Or: add them
1. It cannot be red and green at once.
2. So “or” means add.
✓ Only when they cannot both happen
P(red or green)
= 0.2 + 0.15
= 0.35
step 3 of 4
With algebra
1. Everything still adds to 1.
2. Form the equation, then solve it.
✓ Check: 0.4 + 0.2 + 0.4 = 1
P: 0.4x2x
0.4 + x + 2x = 1
3x = 0.6
x = 0.2, 2x = 0.4
step 4 of 4
Then expect
1. These questions often finish this way.
2. Probability × number of spins.
✓ Expect about 60
P(yellow) = 0.3
200 spins
0.3 × 200 = 60 yellows
mutually exclusive —
All outcomes
add up to exactly 1
A or B
add, if they cannot both happen
If your probabilities add to more than 1,
something has gone wrong.
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