The shape of an entrance to a tunnel can be modeled by the graph of the equation y = -0.2x(x - 20) where x and y are measured in feet. On a coordinate plane, the ground is represented by the x-axis. How wide is the tunnel at its base? Explain how you found your answer.
Since we know it is an upside down parablola, the two points where it crosses the x axis (where y=0) will define the width
y = 0 = -0.2 x (x-20) means x = 0 and x = 20 are the places where it crosses the x-axis (ground level)
distance between 0 and 20 = 20 ft