Puzzle of the day · No. 20 · 1 November
Rain at the weekend
The forecast gives a 50% chance of rain on Saturday and 50% on Sunday, one independent of the other. Lucky has already got his umbrella out. What is the chance it will be needed on at least one of the days?
Answer: 75%. It is easier to count the opposite: both days dry is 0.5 × 0.5 = 25%. Everything else has at least one rainy day, 75%. Not 100%, however tempting “fifty plus fifty” may be.
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