## Monday, February 27, 2006

### An oldie but a goodie

A man traveled by car to a destination 25 miles away. The journey was made between 7 and 9am, so the roads were congested and progress was slow. The journey took 75 minutes, which means that his average speed was 20 miles per hour. He took the same route on the return and traveling in the middle of the day made faster time: his return journey lasted just 25 minutes, meaning that his average speed fro the return journey was 60 miles per hour.

What was his average speed for the two journeys combined?

1. It took him 100 total minutes to go 50 miles so I will say. 30MPH

2. The correct answer is 30mph. Most people's instinct is to add the 20mph and the 60mph and divide by two (20 + 60 = 80. 80 ÷ 2 = 40), which is the way most averages are calculated. But this is not the way to calculate the average of two speeds over a given distance. As Karnov said, the correct answer can only be found by adding the distances, then adding the times, and then calculating a new average: ie., 25 miles + 25 miles = total distance of 50 miles. 75 minutes and 25 minutes = total time of 100 minutes. 100 minutes = 1.6666 (recurring) hrs. 50 miles divided by 1.6666 hrs = 30 mph.

Leave your answer or, if you want to post a question of your own, send me an e-mail. Look in the about section to find my e-mail address. If it's new, I'll post it soon.

Please don't leave spam or 'Awesome blog, come visit mine' messages. I'll delete them soon after.

Enter your Email and join hundreds of others who get their Question of the Day sent right to their mailbox

The Lamplight Manor Puzz 3-D
Are you looking for a particular puzzle, riddle, question, etc? Or do you want to find the answer today rather than wait till tomorrow!

 Web statspuzzles.blogspot.com