A "Tank" Problem

Water is run into a large tank through a hose at a constant rate. After 5 minutes a hole is opened in the bottom of the tank , and water starts to flow out.
Initially the flow rate through the hole is twice as great as the rate through the hose, but as the water level in the tank goes down, the flow rate through the hole decreases; after another 10 minutes the water level in the tank seems to be constant.......
From: Hughes-Hallett, et al., Calculus, 2nd ed.

\(Needs["Graphics`Master`"]; 
Needs["Calculus`Master`"]; \)

g[t_] = -UnitStep[t - 5] (5 + 5 Exp[-1/2 (t - 5)])

Plot[{5, g[t]}, {t, 0, 20}, PlotRange -> {-11, 6}, 
      PlotStyle -> {{AbsoluteThickness[3], Blue}, {AbsoluteThickness[3], 
            Red}}, Ticks
         -> {Automatic, {{5, c}, {-5, -c}, {-10, -2 c}}}, 
      AxesLabel -> {"time (min.)",  "rate (gallons/min)"}];

i[t_] = Integrate[5 + g[s], {s, 0, t}]

Plot[i[t], {t, 0, 20}, 
      PlotStyle -> {AbsoluteThickness[2], LightBlue}, 
      AxesLabel -> {"time (min.)", "vol. (gallons)"}];

container = 
      Graphics[{AbsoluteThickness[3], 
          Line[{{-60, 30}, {-60, 0}, {60, 0}, {60, 30}}]}];

Show[container];

water[t_] = 
      Graphics[{{LightBlue, 
            Polygon[{{-60, i[t]}, {-60, 0}, {60, 0}, {60, i[t]}}]}, {
            Text[StringForm["time = `` minutes", t], {0, 1}, {0, 
                -1}]}}];

Table[Show[water[t], container], {t, 0, 20}];

Helmut Knaust, 3-8-1999