Posted by Anonymous on .
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,
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)=14080
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=140100
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)