Calculate Probability from Outcomes

SECTION 1

Where Did Probability Come From?

People have been playing games of chance for thousands of years. Ancient dice made from animal bones have been found in archaeological digs around the world. But for a long time, nobody had a mathematical way to figure out the probability (the chance) of winning or losing.

It wasn't until the 1600s that two French mathematicians started writing letters to each other about gambling problems. Their work turned the idea of "luck" into actual math. Let's see how it happened!

3000 BCE

Ancient Dice Games

People in Mesopotamia and Egypt played games with dice made from bones. They relied on gut feelings and superstition, not math.

1654

Pascal & Fermat's Letters

Blaise Pascal and Pierre de Fermat exchanged letters about a gambling question. They created the first rules for calculating probability.

1718

De Moivre's Textbook

Abraham de Moivre published one of the first probability textbooks. It organized the ideas into formulas students could learn.

Today

Probability Everywhere

Probability is used in weather forecasts, sports statistics, video games, and even medicine. It shows up on the ISEE too!

The big question probability answers is simple: How likely is something to happen? On the ISEE, you'll be asked to calculate that likelihood using counting. Let's learn how.

SECTION 2

Core Principles of Probability

Before you can calculate probability, you need to know a few key vocabulary words. These terms show up all the time on the ISEE, so learning them now will save you time on test day.

Outcome

A single possible result. For example, rolling a 4 on a die is one outcome. Getting heads on a coin flip is another.

Sample Space

The complete list of every possible outcome. A standard die has a sample space of {1, 2, 3, 4, 5, 6} — that's 6 outcomes total.

Event

The specific outcome or group of outcomes you care about. "Rolling an even number" is an event that includes outcomes 2, 4, and 6.

Favorable Outcomes

The outcomes that match your event. If your event is "rolling a number greater than 4," the favorable outcomes are 5 and 6 — that's 2.

Equally Likely

Each outcome has the same chance of happening. A fair coin and a fair die are equally likely. A rigged game is not.

✦ KEY TAKEAWAY

Think of probability like a pizza. The whole pizza is the sample space (all possible outcomes). The slices you want are the favorable outcomes. Probability tells you what fraction of the pizza you get!

SECTION 3

Seeing Probability in Action

Let's look at a spinner divided into equal sections. The diagram below shows a spinner with 8 equal sections, each a different color. We can use it to understand how probability works visually.

This spinner has 8 equal sections. Since each section is the same size, every color has an equal 1 out of 8 chance of being landed on. That means P(Red) = 1/8, P(Blue) = 1/8, and so on.

What if someone asks: "What is the probability of landing on a warm color (red, yellow, or orange)?" You count the favorable outcomes — that's 3. The total outcomes are 8. So the probability is 3/8. It's really just counting and making a fraction!

SECTION 4

The Probability Formula

Here is the one formula you need for probability on the ISEE. It's short, and once you practice it a few times, it will feel automatic.

PROBABILITY FORMULA

P(event) = Number of favorable outcomes ÷ Total number of outcomes

P(event) means "the probability of the event happening." The favorable outcomes are the ones you want. The total outcomes are all the possible results.

Probability is always a number from 0 to 1. You can also write it as a fraction, a decimal, or a percent. On the ISEE, answers are usually fractions or percents.

PROBABILITY RANGE

0 ≤ P(event) ≤ 1

A probability of 0 means the event is impossible. A probability of 1 means it is certain to happen. Most probabilities fall somewhere in between.

CONVERTING TO PERCENT

Percent = P(event) × 100

To turn a probability fraction into a percent, divide the numerator by the denominator, then multiply by 100. For example, 3/8 = 0.375 × 100 = 37.5%.

💡 ISEE Tip

Always simplify your fraction! If you get 4/8, reduce it to 1/2. The ISEE answer choices almost always use simplified fractions. If your answer doesn't match any choice, try simplifying.

SECTION 5

Common ISEE Probability Setups

On the ISEE, probability questions come in a few common forms. You might see dice, coins, spinners, bags of marbles, or decks of numbered cards. The diagram below shows the most popular setups and their total outcomes.

These five setups appear most often on the ISEE. For every one, the strategy is the same: count the favorable outcomes, count the total outcomes, and make a fraction.

No matter which setup you see, the steps are always the same. First, figure out the total number of outcomes. Second, count how many outcomes match what the question asks for. Third, write the fraction and simplify.

SECTION 6

Step-by-Step Worked Example

Let's walk through a full problem the same way you would on the ISEE. Take it one step at a time.

📝 Sample Problem

A bag contains 4 red marbles, 6 blue marbles, and 2 green marbles. If you pick one marble at random without looking, what is the probability that it is blue?

Finding P(blue marble)

Step 1 — Find the Total Number of OutcomesAdd up every marble in the bag. There are 4 red + 6 blue + 2 green.

Total outcomes = 4 + 6 + 2 = 12

Step 2 — Count the Favorable OutcomesThe question asks about blue marbles. How many blue marbles are in the bag?

Favorable outcomes = 6

Step 3 — Write the FractionPlace the favorable outcomes on top and the total outcomes on the bottom.

P(blue) = 6/12

Step 4 — SimplifyBoth 6 and 12 are divisible by 6. Divide the top and bottom by 6.

P(blue) = 6 ÷ 6 / 12 ÷ 6 = 1/2

Step 5 — Check Your AnswerDoes 1/2 make sense? Half the marbles in the bag are blue (6 out of 12). Yes, so the probability of picking a blue marble is 1/2, which equals 50%. You'd look for 1/2 among the answer choices.

🎯 ISEE Strategy

On the ISEE, always do a quick sanity check. If more than half the items are blue, the probability should be more than 1/2. If fewer than half are blue, it should be less than 1/2. This helps you eliminate wrong answer choices fast.

SECTION 7

Common Mistakes & How to Avoid Them

Even strong math students make predictable mistakes on probability questions. Knowing what these mistakes look like will help you avoid them on test day.

Watch out for these four traps on ISEE probability questions.

Common Mistake	What Goes Wrong	How to Fix It
Forgetting to count ALL items	You count only the colors mentioned in the question, not the total in the bag.	Always add up EVERY item to get the total outcomes, even items not in the question.
Flipping the fraction	You put the total on top and the favorable on the bottom (like 12/6 instead of 6/12).	Remember: favorable on TOP, total on BOTTOM. Probability is never greater than 1.
Not simplifying	Your answer is correct but doesn't match any choice because it's not reduced.	Always simplify your fraction. Check if the top and bottom share a common factor.
Answering with a probability > 1	You accidentally write something like 8/5, which is impossible for probability.	If your answer is greater than 1, something went wrong. Go back and recount.

✦ KEY TAKEAWAY

Think of probability like a basketball free-throw percentage. If a player makes 7 out of 10 shots, the probability of making the next one is 7/10 = 0.70 = 70%. The makes go on top, and the total attempts go on the bottom. Probability can never be more than 100%!

SECTION 8

Connecting to Bigger Ideas

The probability formula you've learned is called theoretical probability because it's based on what should happen. In the real world, there's also experimental probability, which is based on what actually happened during trials. You don't need experimental probability for the ISEE Middle Level, but it's good to know the difference.

Feature	Theoretical Probability (ISEE Level)	Experimental Probability (Future Topic)
Based on	Counting possible outcomes	Collecting data from actual experiments
Formula	Favorable ÷ Total possible	Times it happened ÷ Total trials
Example	P(heads) = 1/2 for a fair coin	Flipped 100 times, got 47 heads → 47/100
On the ISEE?	Yes — this is what they test	Not on the Middle Level exam

As you move into more advanced math, you'll also learn about compound probability — the chance of two events happening together, like flipping heads AND rolling a 6. For now, focus on single events. Master the basics and the harder stuff will come naturally later!

SECTION 9

Practice Problems

Time to practice! These five problems go from easier to harder, just like the ISEE does. Remember: there's no penalty for wrong answers on the ISEE, so always make your best guess. Try each one before reading the answer.

A jar contains 5 red gumballs and 3 yellow gumballs. If you randomly pick one gumball, what is the probability of picking a red gumball?

A standard number cube (die) is rolled once. What is the probability of rolling a number less than 3?

A spinner has 10 equal sections numbered 1 through 10. What is the probability of spinning an odd number?

A bag holds 3 red marbles, 7 blue marbles, and 5 white marbles. One marble is chosen at random. What is the probability that it is NOT white?

A box contains cards numbered 1 through 20. If one card is drawn at random, what is the probability that the number on the card is a multiple of 3?

SUMMARY

Probability — Quick Review

Probability measures how likely an event is to happen. To find it, use the formula: P(event) = favorable outcomes ÷ total outcomes. An outcome is a single possible result, while the sample space is the complete list of all possible outcomes. The favorable outcomes are the specific ones that match what the question asks for.

Probability is always a number between 0 (impossible) and 1 (certain). On the ISEE, always simplify your fraction so it matches an answer choice. For "NOT" questions, count everything except the unwanted item. When in doubt, list out every outcome and count them carefully. You've got this!