<em>w(t)</em> tells you the amount of water <em>w</em> at a given time <em>t</em>.
The inverse function tells you at which time <em>t</em> the tank contains an amount of water <em>w</em>.
To get the inverse function, solve the equation
<em>w</em> = 80 - 2.8<em>t</em>
for <em>t</em> :
<em>w</em> = 80 - 2.8<em>t</em>
2.8<em>t</em> = 80 - <em>w</em>
<em>t</em> = (80 - <em>w</em>) / 2.8
<em>t</em> ≈ 28.57 - 0.357 <em>w</em>
The graph of <em>w(t)</em> 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 <em>w(t)</em>.