To start off, you need to know about dependent and independent events. Independent events are events where the probability of one happening doesn't affect the other. Take flipping two coins for example. Whether you get heads or not on the first one won't affect the probability of getting heads on the second. Dependent events are different, because there is a correlation between. For example, if you walked up to someone on the street, there's a 50% chance that they are female (P(f)=1/2). Now you ask them if they work in childcare. They are more likely to work in childcare if they are female than if they are male. The two events are not independent. This is very important in the Ultra-Mathsy Solution!