Mathematics For Dummies?

Click here for a 15-article collection of articles by Steven Strogatz on mathematics for you and me.

Before going on vacation for a week, you ask your spacey friend to water your ailing plant.  Without water, the plant has a 90 percent chance of dying.  Even with proper watering, it has a 20 percent chance of dying.  And the probability that your friend will forget to water it is 30 percent.  (a) What’s the chance that your plant will survive the week?  (b) If it’s dead when you return, what’s the chance that your friend forgot to water it?  (c) If your friend forgot to water it, what’s the chance it’ll be dead when you return?

Although they sound alike, (b) and (c) are not the same.  In fact, the problem tells us that the answer to (c) is 90 percent.  But how do you combine all the probabilities to get the answer to (b)?  Or (a)?

Read the article Chances Are, on probability theory, for the answers. There's also discussion of a couple of other problems -- one involving percentages concerning the incidence of breast cancer among women, and another involving the O.J. Simpson murder case.