Posted by **Anonymous** on Wednesday, December 7, 2011 at 9:53pm.

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 -
**Allison**, Wednesday, December 7, 2011 at 10:06pm
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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