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K bought some marbles and gave half of them to L. bought some stamps and gave half of them to K.

K used 5 stamps and L gave away 11 marbles. The ratio of the number of stamps to the number of marbles K had left then became 1:7 and the ratio of the number of stamps to the number of marbles L had left became 1:5.

How many stamps did L buy?

  • math -

    K's marbles started at km, and stamps at ks
    L started with lm marbles and ls stamps.

    after paragraph 1,

    K has km/2 marbles and ks+ls/2 stamps
    L has lm+km/2 marbles and ls/2 stamps

    After spending and charity,

    K has km/2 marbles and ks+ls/2-5 stamps
    L has lm+km/2-11 marbles and ls/2 stamps

    Finally,

    (ks+ls/2-5)/(km/2) = 1/7
    (ls/2)/(lm+km/2-11) = 1/5

    Now, having four unknowns and only two equations, I must assume that K started with no marbles (ks=0), and L started with no stamps (lm=0). That leaves us with

    (ls/2-5)/(km/2) = 1/7
    (ls/2)/(km/2-11) = 1/5

    Clear fractions and rearrange terms to get

    7ls - km = 70
    5ls - km = -22

    ls=46
    km=252

    so, K bought 252 marbles, L bought 46 stamps

    check:
    after initial giving,
    K and L each had 126 marbles and 23 stamps

    Then K had 18 stamps and 126 marbles
    and L had 23 stamps and 115 marbles

    18/126 = 1/7
    23/115 = 1/5

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