Pop Quiz

Posted on November 5, 2013 by F. E. Guerra-Pujol

From the math/science section of the G.E.D. high-school equivalency exam:

Assume the probability of rain on each of the next three days is as follows:

Tuesday: 30%

Wednesday: 45%

Thursday: 50%

Based on the above probabilities, what is the probability that it will rain on all three days?

The mathematically-correct answer may surprise you.

About F. E. Guerra-Pujol

When I’m not blogging, I am a business law professor at the University of Central Florida.
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8 Responses to Pop Quiz

  1. The Professor's Wife says:
    November 5, 2013 at 17:58

    What’s the mathematically correct answer? I guess its not the mean? So is it the mode or the median or what?

    Reply
  2. enrique says:
    November 6, 2013 at 00:57

    Careful (or “ojo” as we say in Spanish) — the question does not ask for the average probability over the three-day period but rather for the total probability that it will rain on all three days

    Reply
  3. The Professor's Wife says:
    November 6, 2013 at 02:24

    Because all the values are 50% or less on all three days I think that the probability is 0 that it will rain on all three days. I think it may only rain on one day.

    Reply
    • enrique says:
      November 6, 2013 at 03:43

      Getting “warmer” … the correct answer approaches zero, but it is not zero (it’s around 0.06), since there is some small positive probability that it will rain on each day

      Reply
  4. The Professor's Wife says:
    November 6, 2013 at 16:31

    How do we get to that number? What’s the formula?

    Reply
  5. Park Chapman says:
    November 6, 2013 at 16:49

    If I’m not mistaken, the correct formula is P(A)*P(B)*P(C), or P(ABC). Substituting in the values, it’s .3*.45*.50=.0675.

    Reply
  6. The Professor's Wife says:
    November 7, 2013 at 02:38

    Thank You!

    Reply

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