Question

Starting from 100 feet away, your friend skates toward you and then passes by you. She skates at a constant speed of 20 ft/sec. Write an equation to show her distance d from you in feet after t seconds. At what times is she 40 ft from you?

Answer 1

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:

-60 ft = -(20 ft / sec)t, and thus t = 3 sec.

Answer 2

80 because you would take them all away subtract all of to get the solve