Calculating Simple Probability
Calculate the probability of a simple event with equally likely outcomes using the formula: probability = number of favourable outcomes ÷ total number of possible outcomes; express the result as a fraction in its simplest form; apply to rolling dice, drawing from bags, and other simple chance situations
What a learner can do afterwards
- Calculate the probability of drawing a blue marble from a bag of 3 blue and 7 red as 3/10
- Use the formula P(event) = favourable outcomes ÷ total outcomes to solve at least three different problems
- Explain why increasing the number of favourable outcomes increases the probability
1 · Read
You already know probability sits somewhere between 0 and 1, and that equally likely outcomes all have the same chance. Knowing the exact number helps you decide what to expect when you pick. The formula is: probability = number of favourable outcomes ÷ total number of possible outcomes.
Priya has a box of 3 yellow counters and 9 green counters, 12 counters altogether. She wants the probability of picking yellow. Favourable outcomes: 3 (yellow counters). Total outcomes: 12 (all counters). So P(yellow) = 3/12. Both 3 and 12 divide by 3, so the simplest form is 1/4.
The more favourable outcomes you have, the bigger the top of the fraction gets compared to the total, so the probability climbs closer to 1. Picking from 6 red counters out of 10 gives a bigger probability than picking from 3 red counters out of 10. With two dice, different totals have different numbers of pairs behind them. List the pairs that make each total and count both lists. The total with the longer list is more likely.
Always write your fraction in its simplest form. Check whether the top and bottom numbers share a common factor, like 2, and divide both by it until you can't simplify anymore.
Probability = favourable outcomes ÷ total outcomes, written as a fraction in its simplest form.
Both probability readings were written for this dataset. Existing problems and math-design question credits and the academy video credit are retained.
2 · Watch
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Where it sits
Where this leads
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16 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.