Jack and Christina are standing 150 feet apart on a level surface. Their dog, Lindy, is standing next to Christina. At the same time, they all begin moving toward each other. Jack walks in a straight line toward Christina at a constant speed of 7 feet per second and Christina walks in a straight line toward Jack at a constant speed of 8 feet per second. Lindy runs at a constant speed of 10 feet per second from Christina to Jack, back to Christina, back to Jack, and so forth. What is the total distance, in feet, that Lindy has traveled when the three meet at one place?  Pick:
A) 40 B) 60 C) 80 D) 100 E) 120

Rationale: "the relative speed of jack and christina is 7 + 8 = 15 feet per second . the distance between them is 210 feet , hence they will meet in ( time ) = ( distance ) / ( relative speed ) = 150 / 15 = 10 seconds . for all this time lindy was running back and forth , so it covered ( distance ) = ( speed ) * ( time ) = 10 * 10 = 100 feet . answer : d ."
Answer: D