Hello, reader! If you intend to post a link to this blog on Twitter, be aware that for utterly mysterious reasons, Twitter thinks this blog is spam, and will prevent you from linking to it. Here's a workaround: change the .com in the address to .ca. I call it the "Maple Leaf Loophole." And thanks for sharing!

Tuesday, May 19, 2009

Wait! What?

There is a 75% chance of rain on any given day in the next week. What is the probability that it will rain on at least 5 of the 7 days?

There's a little surprise in there. I won't spoil the fun. (Until I do. Probably tomorrow.)

Update (also in comments): The weird thing, that you might notice in the course of solving, if you calculate P(5), P(6), and P(7) separately and then add them together, is that P(5) and P(6) come out to the same thing. The probability it will rain 5 out of the 7 days is the SAME as the probability it will rain 6 out of the 7 days. Isn't that weird? If you write it out with fractions and the nCr formula you can how it happens, but the result is still counterintuitive to me. Combinations sometimes surprise me like that.