Basic introduction to probability

Surprisingly the basic introduction to probability was found in a biology book!
Here is the summary:
Product Rule:
When two or more events are independent, the probability that they will occur in succession is calculated using the product rule--their individual probabilities are multiplied. Ex : probability of getting two heads in succession is 1/2 x 1/2 = 1/4

Sum Rule:
Sum Rule is applied when there are two or more different ways of obtaining the same outcome; that is, the probability tat either event A or event B will occur equals P(A) + P(B)

Source: Billogy: Te dynamic science by Peter Russell p.240