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Learn the skill
For a conditional probability, restrict attention to the group named after 'given.' Count favorable outcomes within that group, then divide by that group's total. Do not automatically use the total sample size.
Worked example
If 3 of 5 blue tiles are round, the probability of round given blue is 3/5, even if a box contains many other colors.
An Invented Tile Box
Red round: 6. Red square: 2. Blue round: 4. Blue square: 8. Select one tile uniformly at random from the stated group.