Your friend's distance from her starting point to your position is decreasing until she passes by you. This distance is D(t) = 100 ft - (20 ft / sec)t. We'll assume that your friend doesn't actually pass by you; she just gets closer and closer, from her initial 100 ft distance from you. Then:
D(t) = 40 ft = 100 ft - (20 ft / sec)t. Simplifying, we get:
