On Decemeber 18th, there is a 75% chance that Santa will come to the mall that day and a 55% chance that your best friend will be at the mall. There is a 40% chance that your best friend will be there AND Santa will come to that mall. What is the probability that Santa will come to the mall OR your best friend will be there?
+23246 Since there is a 75% chance that Santa will be at the mall and there is a 40% chance that both Santa and your best friend will be at the mall, there is a 75% - 40% = 35% chance that Santa will be at the mall but your best friend won't be.
Since there is a 55% chance that your best friend will be at the mall and there is a 40% chance that both Santa and your best friend will be at the mall, there is a 55% - 40% = 15% chance that your best friend will be at the mall but Santa won't be.
Add these three mutually exclusive events together:
(only Santa is at the mall) + (both Santa and your best friend is at the mall) + (only your best friend is at the mall)
= 35% + 40% + 15% = 90%
