two queations in princeton math

lll123

1.bushs leave town at 3 pm and every 10hours after that. bushs leave town c at 4 pm and every 15hours after that. if the buses follow this schedule beginning on a monday, what is the earliest day on which the buses leave at the sametime?

answer is "so long as they continue operating in this manner,they will never leave at the same time." explanation is "if you call noon on monday zero,then buses leaving town b depart at 3,13,23,33,43,53,etc.(always ending in 3).buses depart from town c at 4,19,34,49,64,79,etc(always ending in 4 or 9),so the buses will never depart at the same time.

my question is "always ending in 3" and "always ending in 4 or 9" mean what? how conclude " so the buses will never departat the same time"

2.还有一道题的问题是“what's the x-coordinate of point b?",它与“b's horizontal distance from point a"的区别是什么？（a与b是两条平行线上的两个完全不同的点）

mariezhu

第二题前者问的是b的横坐标是多少,

后者问的是ab两点,b的横坐标减a的横坐标是多少