Marbles Knives
Two probability equations?
Blindly, mindlessly writing down some probability definitions/equations on my math exam’s memory aid, I forgot to title one of them and can’t figure out the difference between the two. When do I use each one?
The first, untitled one:
The probability of an event is equal to the sum of probabilities of simple events.
E.g. A drawer contains 8 knives, 10 forks, and 12 spoons. What are the chances of taking out a knife or fork?
8/30 + 10/30 = 18/30 = 60%.
Here’s the second one:
Random Experiment with several steps;
In a random experiment with several steps, the probability of an event is equal to the product of probabilities at each step.
E.g. Numbered 1-10, ten marbles are in a bag. What are the chances of drawing a 7 and then a 3?
1/10 x 1/10 = 1/100 (1%)
So when do I use the first, and when do I use the second?
The first is an “or” situation, chance of this or that happens, where you add fractions
The second is “and”, where you multiply fractions
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