speed of a moving object can be represented by the slope of a line
where the vertical displacement is the distance travelled and the
horizontal displacement is the time elapsed. Thus, a triangle can
be drawn for each part of the trip. Since the distance up is the
same as the distance down, the heights of the two triangles are
The dotted line starting from the left vertex represents the average
speed of the elevator for the two trips.
can be replaced with d and ,
dotted lines at the right of the figure complete the triangle to show
that the unknown speed is the ratio of 2d to .
slope of the dotted line is found by simplifying the expression .
This results in a speed of
feet per second. Changing this to feet per minute yields 90 ft/min,
the solution to the problem.