How do you find the probability of something?

Sometimes you can make a practical assumption about the probability of something occurring by considering all the possible outcomes. For example, a coin only has two sides so the probability of it landing on heads (assuming that the coin is "fair") is 1/2. Similarly, a die only has six sides so the probability of it landing on a 4 is 1/6. At other times it may not be possible to make any assumptions about the possible outcomes. In those situations you may have to estimate the probability by measuring how many "successes" you get as a proportion of how many "trials". Say, for example, you want to estimate the probability of a yellow truck going down the road. You could sit by the side of the road for a day and measure the number of yellow trucks that pass by (successes) as a proportion of all traffic (trials).