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11 April, 17:02

The Lees have 3 children. The oldest is twice as old as the youngest. The middle is 5 years older than the youngest. If the sum of the age is 57, how old is each child?

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  1. 11 April, 18:42
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    Oldest = 2 times Youngest - > O = 2*Y

    Middle = Youngest + 5 - > M = Y+5

    All of them together is 57 - > O + M + Y = 57

    So you have these three equations:

    (1) O = 2*Y

    (2) M = Y+5

    (3) O + M + Y = 57

    Now you want to reduce the number of variables. You can change the second equation to be Y = M-5 and then plug in "M-5" wherever there is currently a Y:

    (4) O = 2 * (M-5) = 2*M - 10

    (5) O + M + (M-5) = 57

    which becomes O + 2M = 62

    Then you plug in the "O" equation (4) into (5) which gives you

    (2M-10) + 2M = 62 which reduces to 4M = 72.

    So now I know M is 18.

    I can now plug that into my other equations:

    (4) O = 2*18 - 10 which means O = 26.

    Now I plug that into (1) from the top:

    26 = 2*Y which becomes 13 = Y

    So now I have O, Y, and M

    Oldest is 26

    Middle is 18

    Youngest is 13

    Reading the sentence again, you can see that this makes sense.
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