How to Calculate Average Speed for a Commuter?

[Andyjimbo]

[SOLVED] Average speed

Hi, Please could someone enlighten me as to how to solve the following, given that the total time of the journey is not given?

A commuter drives to work 14 miles. Due to traffic, the commuter averages 25 miles per hour on the first 7 miles of the drive. How fast must the commuter go on average on the second half to average 40 miles per hour for the entire drive?

Many thanks

 

[Hootenanny]

Note that 7 miles is half of 14 miles...

 

[Andyjimbo]

Thank you for your reply. I had noticed that! However, given that 55mph is the wrong answer I still lack the tools to finish the job. Please could you tell me how to solve this, then I'll be equipped for life :)

 

[physixguru]

this should help ya...:

Louise runs the first half of a race at 5 miles per hour. Then she picks up her pace and runs the last half of the race at 10 miles per hour. What is her average speed on the course?
A The answer is not 7.5 miles per hour. Assume the course is 20 miles long. Louise needs 2 hours to run the first 10 miles (the first half of the course) and 1 hour to run the last 10 miles (the last half), meaning she needs 3 hours to run the total of 20 miles. Therefore, her average speed is 6.7 (or 6 2/3) miles per hour.

 

[Andyjimbo]

Okay - So:

7 miles at 25mph = 16min 48 secs

14 miles at 40mph = 21 mins

21mins - 16.8 mins = 4.2mins

So the driver must do:

7miles in 4.2 mins = 100 mph (99.99999)

to average 40mph for the whole journey. Am I right?

 

[physixguru]

what do you think...?

 

[Andyjimbo]

I think I love ya... :0

 

[physixguru]

keep loving me...:p

U are right dear...

ENJOY!