8th grade math

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A fish tank weighs 80 pounds when 40% full of water, and it weighs 140 pounds when completely full. How many pounds does the tank weigh when empty?

  • 8th grade math -

    Okay, so you have two variables, let's call them x and y. x will be the weight of the water when the tank is full, and y will be the weight of the tank.
    So you can get two equations from the question:
    .4x+y=80 (1)
    x+y=140 (2)
    and now you can do elimination by subtracting (1) from (2).
    x+y-(.4x+y)=140-80
    so you get
    .6x=60
    then divide by .6 to get
    x=100
    and since you want y, sub x=100 into (2)
    100+y=140
    y=140-100
    y=40
    So the tank weighs 40 pounds when empty.
    Sorry if this is a little long (I wasn't sure how much detail is too much, you see)

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