There are 12 inches in one foot. 120 x 12 = 1,440 inches. The correct ratio to be placed is the 5th one (aka 12 inches over 1 foot).
Rearrange slightly...
12xy-8y-9x+6 now factor 1st and 2nd pair of terms
4y(3x-2)-3(3x-2)
(4y-3)(3x-2)
If each week the tank decreased by 3 inches, in 2 weeks it would be 6 in, 3 weeks is 9 in, 4 weeks would be 12 in, and 5 weeks would be 15 in.
Let d(x) = 2x - 4
Or
y = 2x - 4
We have replace x = y
x = 2y - 4
Now Isolate "y"
x + 4 = 2y
Pass "2" dividing
(x + 4) / 2 = y
y = x/2 + 2
Or
d(x)^-1 = x/2 + 2
