Probability question

1. Apr 12, 2005

EvilPony

There is a 80% chance that Wednesday's mail will be delivered before noon on Wednesday and a 40% chance that Thursday's mail will be delivered before noon on Thursday. If these probabilities are independent, what is the probability that the mail will arrive before noon at least one of those two days?

2. Apr 12, 2005

Hurkyl

Staff Emeritus
What thoughts have you had so far?

3. Apr 12, 2005

EvilPony

heh guess I should be posting what ive done on these instead of just posting the question :)

ok well all I know is that its .8 (x) + .4 (x) , but I have no idea what the variable x is. I cant just add .8 and .4 because that would be greater then one. So i guess I just need help figuring out what x is

4. Apr 12, 2005

Hurkyl

Staff Emeritus
Well, why do you think that?

5. Apr 12, 2005

EvilPony

or maybe this has something to do with the independent test...

6. Apr 12, 2005

Moo Of Doom

One way to go about solving "at least one" problems, is to think of the probability that none happen.

7. Apr 12, 2005

EvilPony

so 1-.2(.6)? which is 88% then?

8. Apr 13, 2005

jdavel

Evilpony,

Does that answer (88%) seem to make sense? Should it be greater than the probability on either individual day?