27.. A streetlight is on top of a 20-foot pole. A person who is 5 feet tall walks away from the pole at the rate of 5 feet per second. At what rate is the tip of the person's shadow moving away from the pole when he is 20 feet from the pole?

29. Helium is pumped into a spherical balloon at a constant rate of 4 cubic feet per second. How fast is the radius increasing after 1 minute? After 2 minutes? Is there any time at which the radius is increasing at a rate of 100 feet per second? Explain.

31.. A point is moving on the graph of y= e^x+x+1 in such a way that itsxcoordinate is always increasing at a rate of 3 units per second. How fast is they coordinate changing when the point crosses the x axis?