- Fair weather predicted and occurred: probability 1/2 * 2/3 = 1/3.
- Fair weather predicted but rain occurred: probability 1/5 * 1/3 = 1/15.
- Rain predicted and occurred: probability 4/5 * 1/3 = 4/15.
- Rain predicted but fair weather occurred: probability 1/2 * 2/3 = 1/3.

If she never takes her umbrella, the inconvenience will amount to 2*i*
one day out of three, for an average inconvenience again of 2*i*/3.

If she decides to follow the advice of the weather forecasters and takes her
umbrella only when rain is predicted why will suffer an inconvenience of
*i* on 1 day out of 3, and i*i* on 1 day out of 15, which amounts to
and average penalty of 7/*i*/15 (less than the two previous average
penalties). We recommend, therefore, that Francine follow the advice of the
weather forecasters.

Source: Berrondom, Marie

Categories: Probability, Reasoning

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