Part I. How we get the formula for multiple trial Probability
If you're on this page, you hopefully know that if you flip a coin you have a ½ probability of getting heads.
But let's consider the following situation. What if we flip the coin twice. What is the probability that we will get heads both times. This is an independent event because the first event, the coin toss, does not effect the second event, the second toss.
The probability of these two independent events is ¼!
Let's look at the sample space for these tosses
First Toss  Second Toss 



What about three independent events?
What is the probability that we will get heads exactly three times if we toss the coin three times?
The probability of getting heads all three times is
Let's look at the sample space for these tosses:
Three Ways that we can get 1 Heads out of three tossesDeveloping the Formula
p(odd)  p(even)  p(even)  p(even) 
To calculate the probability of independent events simply multiply each probability together
× × × = =Imagine that we are using the same spinner depicted up above. Calculate the probability of obtaining exactly 1 odd number on 4 spins of the arrow.
Probability  
O  E  E  E  
E  O  E  E  
E  E  O  E  
E  E  E  O 
To determine the total probability of these independent events add up each fraction: . Note, this is the same as . Quick refresher on the formula for combinations in math.
Formula for Probability of Independent Events
Formula = The general formula is to determine how many combinations of the independent events can occur, then multiply the probability of each by the result of the combination. It's much easier to understand by looking at some more examples.