Mary drove at an average speed of 40 mph for some time and then at an average speed of 60 mph for the rest of the journey. If she made no stops during the trip and her average speed for the entire journey was 50 mph, for what fraction of the total time did she drive at 40 mph?
Select
A) 1/5  B) 1/3  C) 1/2  D) 2/3  E) 3/5
We don't need to get into calculations for solving this question. We can use the concept of weighted averages.   One thing for sure we know that if the average speed for the entire journey is 50 mph, means she drove at 40 mph and 60 mph for an equal duration of time.   10 10   40-----50----60   This shows that you can divide the entire journey into 2 equal parts. Thus, 1/2 parts she drove at 40 mph and 1/2 part she drove at 60 mph.   Answer: 1/2
<result>C</result>
Mary drove at an average speed of 40 mph for some time and then at an average speed of 60 mph for the rest of the journey. If she made no stops during the trip and her average speed for the entire journey was 50 mph, for what fraction of the total time did she drive at 40 mph?