One hour after Yolanda started walking from X to Y, a distance of 24 miles, Bob started walking along the same road from Y to X. If Yolanda's walking rate was 3 miles per hour and Bobт's was 4 miles per hour, how many miles had Bob walked when they met? Select the correct option:
A) 24  B) 23  C) 22  D) 21  E) 12

Rationale: "when b started walking y already has covered 3 miles out of 24 , hence the distance at that time between them was 24 - 3 = 21 miles . combined rate of b and y was 3 + 4 = 7 miles per hour , hence they would meet each other in 21 / 7 = 3 hours . in 6 hours b walked 3 * 4 = 12 miles . answer : e ."
Answer: E