If both buses travel with an average speed of 45 mi/h and we want to find when will they be 400 miles apart, we need to find when one of them will be 200 miles from its start point. To do it let's see how long does it take to make 1 mile:
45 miles per hour = 45/60 mile per minute = 3/4 mile per minute = 0,75 mile per minute
200 ÷ 3/4 =
= 200 * 4/3 =
= 800/3 =
= 266 2/3 [minutes] =
= 4 hours 26 minutes 40 seconds
That's how long it takes for one bus to be 200 miles from its start point. It also means that both buses will be 400 miles apart after 4 hours 26 minutes and 40 seconds of driving opposite directions.
