(And ) (OR)

Question

How do we know in probability question that its AND or OR question. AND = Multiplication OR= addition Please help, i'm messing up simple problems because of this little confusion.

Thanks

skovinsky · Answer

Hi,

if you're calculating the probability of MULTIPLE events occurring, you always MULTIPLY;

if you're calculating the probability of ALTERNATIVE events occurring, you ADD.

Some common multiple event words: "and", "both" and "all".

Some common alternative event words/phrases: "or", "at least" and "at most".

On the toughest probability questions, you may have to do a bit of both. For example:

Since there are two years, there are four possible scenarios for Yohann:

1) Yr 1 housing, Yr 2 housing;
2) Yr 1 housing, Yr 2 no housing;
3) Yr 1 no housing, Yr 2 housing; and
4) Yr 1 no housing, Yr 2 no housing.

We want Yohann to get housing in exactly one of the two years, so scenarios (2) and (3) match our desired outcome; in other words, we want scenario (2) OR scenario (3).

So, we begin by calculating the probabilities of scenarios (2) and (3):

For (2), we want a yes AND a no (so we multiply): (1/3)(2/3) = 2/9
For (3), we want a no AND a yes (so we multiply): (2/3)(1/3) = 2/9

However, we're happy with (2) OR (3), so we find the final answer by ADDING those two probabilities:

(2/9) + (2/9) = 4/9

* * *

Hope that helps!

shrive555 · Answer

THanks a lot Kovinsky

(And ) (OR)

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