 total ways a specific outcome will happen 
Probability = 

 total number of possible outcomes 
Example: There are 87 marbles in a bag and 68 of them are green. If one marble
is chosen, what is the probability that it will be green?
Solution
 Divide the number of ways to choose a green marble (68) by the total number of marbles (87)
 68 ÷ 87 = 0.781609
 Round to the desired precision (e.g. 0.781609 rounded to hundredths is 0.78)