One hour after Yolanda started walking from X to Y, a distance of 31 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?
Choose the correct choice from the following answers.
A) 24	B) 23	C) 22	D) 21	E) 16

Rationale: "when b started walking y already has covered 3 miles out of 31 , hence the distance at that time between them was 31 - 3 = 28 miles . combined rate of b and y was 3 + 4 = 7 miles per hour , hence they would meet each other in 28 / 7 = 4 hours . in 6 hours b walked 4 * 4 = 16 miles . answer : e ."
Answer: E