probability question math

[spoog74]

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If you choose a random number between 21 and 33 (including 21 and 33), what is the probability that it is even or divisible by 3?
answer says it is 9/13 but wouldnt it be 11/13?


I got for odd 6/13 + 5/13 as divisible by 3. Wouldnt that be 11/13?

 

[shulk]

There is a 6/13 chance of it being even. A 5/13 chance of it being divisible by 3. And a 2/13 chance it with be both even and divisible by 3. 6/13+5/13-2/13 = 9/13 chance.

Because there are situations where both conditions are true, you have to reduce the final probability.