--- title: "Calculating Simple Probability" description: "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 fra" canonical: https://lightmysky.com/learn/mathematics/calculating-simple-probability-mt_Zt30Gxi-qp source: https://lightmysky.com/learn/mathematics/calculating-simple-probability-mt_Zt30Gxi-qp.md retrieved: 2026-10-19 --- > **Agent view.** This is the Markdown twin of the page, for tools and assistants. > When to use this site, and the call that answers each job: https://lightmysky.com/agent-instructions.md > API description (OpenAPI 3.1): https://lightmysky.com/openapi.json · Authentication: https://lightmysky.com/auth.md > Pricing: https://lightmysky.com/pricing.md · Catalog: https://lightmysky.com/llms.txt · Full catalog: https://lightmysky.com/llms-full.txt > Every machine-readable file on this domain: https://lightmysky.com/.well-known/ai-catalog.json > Ask for Markdown with `Accept: text/markdown`, a `.md` address, or `?mode=agent`. # 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 Subject: Mathematics · Area: Probability · Ages 10 to 11 Page: https://lightmysky.com/learn/mathematics/calculating-simple-probability-mt_Zt30Gxi-qp ## Ready when they can - 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 ## Lesson: Calculating simple probability 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. *(drawing: A bag has 1 blue marble and 4 red marbles, 5 marbles in total. 1 of the 5 parts is shaded blue, so the probability of picking blue is 1/5.)* **Example.** 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. **Tip.** 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. **Recap.** Probability = favourable outcomes ÷ total outcomes, written as a fraction in its simplest form. ## Practice 16 questions on this page, each with its working shown. ## Needs first - [The 0-to-1 Probability Scale](https://lightmysky.com/learn/mathematics/the-0-to-1-probability-scale-mt_t0g2SlP404) - [Equally Likely Outcomes](https://lightmysky.com/learn/mathematics/equally-likely-outcomes-mt_xSgAgg9Ej_) ## Opens up - [Experimental vs Theoretical](https://lightmysky.com/learn/mathematics/experimental-vs-theoretical-mt_-bMnJcPJy8) - [Probabilities Sum to One](https://lightmysky.com/learn/mathematics/probabilities-sum-to-one-mt_3fwYu7imd4) - [The Probability Scale](https://lightmysky.com/learn/mathematics/the-probability-scale-mt_vmQJAtAFuy)