w(t) tells you the amount of water w at a given time t.
The inverse function tells you at which time t the tank contains an amount of water w.
To get the inverse function, solve the equation
w = 80 - 2.8t
for t :
w = 80 - 2.8t
2.8t = 80 - w
t = (80 - w) / 2.8
t ≈ 28.57 - 0.357 w
The graph of w(t) is a downward-sloping line. The graph of the inverse is also a downward-sloping line, but its intercept (28.57) and slope (-0.357) are both smaller than the intercept and slope of w(t).
