There are 2 types of probabilty: theoretical and experimental. In 7th grade math, you will use both and will have to be able to calculate the probability of an event in fraction and percent form.
Theoretical Probability
Theoretical probability is based on what should happen in a given situation. It is calculated using the number of favorable outcomes divided by the total number of possible outcomes.
For examples, if we flip a coin, and we are wanting the coin to land on heads, then the theoretical probability is 1 out of 2 or 1/2. There is only one side that is heads out of two sides of the coin.
If you need to map out something more complicated, like say all of the possibilities for ordering lunch in the cafeteria, you can use a an outcome tree like the one below:
For examples, if we flip a coin, and we are wanting the coin to land on heads, then the theoretical probability is 1 out of 2 or 1/2. There is only one side that is heads out of two sides of the coin.
If you need to map out something more complicated, like say all of the possibilities for ordering lunch in the cafeteria, you can use a an outcome tree like the one below:
The outcome tree lists all of the possibilities by starting with the initial choice or possible outcomes. From those first choices or outcomes, list the possible outcomes for each choice. Then, for the next set of outcomes or choice, follow the same process. The final column will list the number of possibilities and each sequence shows a different possibility.
Experimental Probability
Experimental probability is based on something that has already happened. It is found using the total number of favorable outcomes that occured divided by the total number of events.
For instance, if I flipped a coin 50 times and got the following results:
Heads: 22
Tails: 28
The experimental probability of flipping a heads is 22 out of 50 or 22/50.
Whenever you have probability, you must always remember to simplify, just as you would for a fraction. So 22/50 can be reduced to 11/25 or 11 out of 25.
For instance, if I flipped a coin 50 times and got the following results:
Heads: 22
Tails: 28
The experimental probability of flipping a heads is 22 out of 50 or 22/50.
Whenever you have probability, you must always remember to simplify, just as you would for a fraction. So 22/50 can be reduced to 11/25 or 11 out of 25.