单选题 One hour after Tom started walking from X to Y, a distance of 45 miles. Bob started walking along the same road from Y to X. If Tom’s walking rate was 3 miles per hour and Bob’s was 4 miles per hour, how many miles had Bob walked when they met?
【正确答案】 A
【答案解析】假设Bob走了n小时，那么(n+1)×3+n×4=45，得到n=6，Bob共走了4×6=24英里，选择A。