algebra

A shipment of 18 cars, some weighting 3000lbs a piece, and the others weighing 5000 lbs each, has a total weight of 30 tons. Find the number of each kind of car.

I start of saying X+Y = 18. THen I turn that into X=18-Y. whats next??
thank you for helping if u can.

asked by Josie
  1. you have to define your x and y

    let the number of heavy cars be x
    let the number of lighter cars be y

    so, ok, at
    x+y = 18
    x = 18-y

    5000x + 3000y = 30(2000)
    divide by 1000
    5x + 3y = 60
    sub in your first part
    5(18-y) + 3y = 60
    90 - 5y + 3y = 60
    -2y = -30
    y = 15

    There were 15 of the lighter cars, and 3 of the heavier cars

    check:
    15(3000) + 3(5000) = 60000 pounds = 30 tons

    posted by Reiny

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