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
The lesson
You already know probability sits somewhere between 0 and 1, and that equally likely outcomes all have the same chance. Now you will turn that idea into an exact number. The formula is: probability = number of favourable outcomes ÷ total number of possible outcomes.
Priya has a box of 4 yellow counters and 6 green counters, 10 counters altogether. She wants the probability of picking yellow. Favourable outcomes: 4 (yellow counters). Total outcomes: 10 (all counters). So P(yellow) = 4/10. Both 4 and 10 divide by 2, so the simplest form is 2/5.
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.
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.
Watch it
Where it sits
Where this leads
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8 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.