How to spot it
What will the question look like?
Look for and, or, without replacement, given that and at least one.
DM topic guide
Probability
Calculate single, combined and conditional outcomes.
Guide page7 of 7
Probability
Calculate single, combined and conditional outcomes.
Rules to remember
Keep these in view
- AND usually means multiply along a path.
- OR adds mutually exclusive routes.
- Without replacement changes the next fraction.
- At least one is often easiest with 1 minus none.
Common traps
What tends to pull students off course
- Adding overlapping outcomes
- Assuming independence
- Reversing conditional probability
Probability in context
From a bag with 3 red and 2 blue, two reds without replacement is 3/5 × 2/4 = 3/10.
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