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Tough Percentage Puzzle

  • 18-11-2009 11:13pm
    #1
    Registered Users, Registered Users 2 Posts: 489 ✭✭


    At college, 70% of the students studied Maths, 75% studied English, 85% studied French and 80% studied German.
    What percentage at least must have studied all 4?


    Initially, you’d think that it would be 70% AND 75% AND 85% AND 80%, which would give you 37.5%, but that doesn’t take into account that doing one subject doesn’t have any effect on whether any of the other subjects are done.

    I’ve seen another solution here, but I don’t find it convincing, though I believe the numerical answer.
    70 +75 + 85+ 80 = 310
    310 ÷ 3 = 100 remainder 10
    3 subjects each student, 10 at least 4

    I’ve been trying to draw Venn diagrams and Karnaugh maps with 4 variables where the outcome is F = ABCD, but I don’t know how to work that with the percentages.

    Help would be much appreciated!


Comments

  • Closed Accounts Posts: 6,151 ✭✭✭Thomas_S_Hunterson


    I think the Inclusion-Exclusion principle should help here.

    /edit: I just stumbled upon a very easy but indirect way to do it.
    It involves looking at the people not studying particular subjects, the exclusion


  • Registered Users, Registered Users 2 Posts: 40,232 ✭✭✭✭Mellor


    look at it this way, the highest is 70%,
    so at least 30% couldn't of done all 4, (30% didn't do Maths)
    consider all subjects this way, what if they wre all different people

    10%


  • Registered Users, Registered Users 2 Posts: 489 ✭✭clartharlear


    You're both marvellous, thanks.


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