Jack rode his bicycle at an average speed of 5 mph for some time and then at an average speed of 15 mph for the rest of the journey. If he made no stops during the trip, and his average speed for the entire journey was 10 miles per hour, for what fraction of the total time did he ride at 15 mph? Choose the correct choice from the following choices.
A) 1 / 5  B) 1 / 3  C) 2 / 3  D) 2 / 5  E) 1 / 2

Rationale: we do n ' t need to get into calculations for solving this question . we can use the concept of weighted averages . the average speed for the entire journey is 10 mph ; so , he rode at 5 mph and 15 mph for an equal duration of time . difference is 5 and 5 , respectively . 5 - - - - 10 - - - - 15 this shows that you can divide the entire journey into 2 equal parts . thus , 1 / 2 of the journey he rode at 10 mph , and 1 / 2 of the journey he rode at 15 mph . answer : e
Answer: E