(a) If she tries her keys at random, keeping aside those that don’t work, what is the

probability that she will open the door on her 10th try?

(b) What if she doesn’t keep aside previously tried keys?

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(a) If she tries her keys at random, keeping aside those that don’t work, what is the

probability that she will open the door on her 10th try?

(b) What if she doesn’t keep aside previously tried keys?

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## Answers

If she puts aside the keys that don't work, then on her tenth try she would have a 1/15 chance of choosing the right key.

If she doesn't put away the keys that don't work, then it's a 1/25 chance every time until she finally chooses the right one.

If you're asking what are the odds of her opening the lock on EXACTLY the tenth try, I'm not sure.

d

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Idk