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Where you can err when calculating probabilities

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  1. Three hunters shoot at the same time at a hare. The first hunter has a hit rate from 3 to 2, second achieves a rate from 3 to 7 and third the one from 1 to 9.
    What is the probability that at least one of the hunters hurts the hare?

  2. Half of all families with two children consists of just as many boys as girls.
    How does it present in families with four children?

  3. Assumed the probability to have an accident on 1 km of motorway is exact 0,0001.
    What is the probability to have an accident on route of 1000 km ?

  4. In a terrible medieval battle 85% of the fighter lost an ear, 80% an eye, 75% an arm and 70% a leg.
    What is the minimum percentage of those, which lost an ear, an eye, an arm and a leg at the same time?

  5. Directly at the start of a dangerous car race track was a very narrow and small bridge from which one of 5 cars fell into the water. Then came a close hairpin curve, in which 3 of 10 cars had an accident. From a dark tunnel only 9 of 10 auto found out. Right before the finish was a sandy runway, on which 2 of 5 cars stucked.
    What is the percentage of the racing cars which had an accident on the whole distance?

  6. If you know that the probability that at least two persons of a group have their birthday on the same day, is lower than ½, but this statement was not correct any more, if a further person would join the group.
    How many persons are in the group?

  7. To my emirate belongs a desert, in whose center I built my palace, and a territorial zone in the sea. Its size amounts to a third of the size of my desert and covers a part of my oil field. I possess a three times as large surface with oil under the desert as sea without oil, and the seventh of my area without oil is in the sea.
    What is the proportion of my oil field under the sea?

  8. With a tribe of natives a ethnological scientist realized that 25% of the persons die with 40 years, 50% with 50 years and 25% 60 years. Subsequently, he picks out at random two persons, whom he wants to study in more detail.
    What is the life expectancy of that one, which will live longest?

  9. 20 air hostesses present themselves as a companion for a flight. 7 are blond, the others are brunette. Finally, three of them are drawn lots.
    What is then the probability to find at least one of each hair colour among them?

  10. It is known that on 250 births comes a twin pair, and that it concerns in three cases once uniovular twins.
    What is the probability that a pregnant woman expects diovular (boy + girl) twins?

  11. In a bathing resort a shark detector is installed, which releases alarm once every 30 days in the average. There are ten times more frequently false alarms than not announced sharks. Additionally only 3 of 4 sharks are discovered with this system.
    What is the percentage of days, on which there are neither false alarms nor not announced sharks?

  12. Paul and Caroline are arranged to meet under the Brandenburger Tor between 11 and 12 o'clock. Everyone arrives in any time in the course of this hour. Then, if they do not meet within the next quarter of an hour, everyone would leave the place.
    What is the probability that the two meet each other in reality?

  13. In a certain area it rains every four days. It is known however that, if it rained on one day it also rains 2 of 3 times on the next day.
    What is the probability that it does not rain on one day, if it also did not rain on the day before?

  14. In a certain holidays place it is stated that 28% of the holiday-maker read "Le Monde", 25% "Le Figaro" and 20% "L'Aurore". Further read 11% at the same time "Le Monde" and "Le Figaro", 3% "Le Monde" and " L'Aurore" and 2% "Le Figaro" and "L'Aurore". 42% read none of these three newspapers.
    What is the percentage of the holiday-makers, who read all three newspapers?

  15. Ten married couples eat together in the evening. Afterwards 5 persons are drawn lots to wash the dishes.
    What is the probability that there is no married couple among them?

  16. Anne and Brigitte are two twin sisters. In the school they divide the same table, whereby the one sits always left and the other one always on the right. They maintain both to be Brigitte. The teacher observes that the right twin sister lies each fourth time and the left each fifth time.
    What is the probability that Anne sits left?

  17. In one year only 3 autolabels participated in the 24 hours of Le Mans: Ford, Jaguar, Maserati. If you give the label Ford a chance of 2 to 2 to win and the label Jaguar a chance of 5 to 1, what would be then the chance for the label Maserati not to win?


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