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The only vocabulary you’ll ever need: “And” and “Or”
There are two ways events can happen together in the same probability problem:
either they could both happen separately or they must happen together.
A) Scenario 1 – “Or”:
If both events do not necessarily
have to occur together, an “or” may be used as in:
I will
be happy today if I win the lottery OR have email.
“OR” means that we add probabilities together to get
a higher overall probability.
Example 4
John will win $100 if, from a deck of 52 standard playing cards, he chooses
either a 7 or a 9 when pulling a single card from the deck. What is the probability
that John will win $100?
Answer:
Start by noticing the word “or” in the question. How can John win?
He can win by pulling out either a 7 or a 9. His chances of doing that are higher
than if he could win only by pulling out a 7. In that case, he’d only
have 4 cards that would make him win $100 (because there are 4 7's in a standard deck), now he has 8 cards. To find the total
probability, we need to figure out the probability of each event and then add
them together.
So, what is the probability of each? Think Bottom to Top:
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