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)