← Day 19
UNIT 7 / 8 · LESSON 3 / 4 · 24 MINUTES

Choose the right denominator

A conditional probability limits the group you are counting. The denominator must match the stated condition.

01 / THE METHOD

Make it repeatable

  1. Name the group after the word “given.”
  2. Count favorable outcomes inside that group.
  3. Divide favorable by total in the restricted group.
  4. Check that the result lies between 0 and 1.
02 / WORKED EXAMPLE

Watch the method in action

Of 20 students, 8 study Math and 5 of those 8 also study science. What is the probability of science given Math?
  1. The condition is studying Math.
  2. There are 8 students in that group and 5 favorable outcomes.
  3. Compute 5/8.

The answer is 5/8, not 5/20.

03 / QUICK CHECK

Try it yourself

Which denominator applies?

04 / PRACTISE AND CHECK

Apply the method to real questions

Start with guided questions and feedback. Then take a fresh independent set without hints. Reach at least 80% on that set to record this lesson.

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