Puzzler of the week

[Scott MacLean]

This week's puzzler:

D-Day is approaching. It�s vitally important that the location of German troops and ammunitions dumps be communicated to the Allied Command in England.  The French Resistance fighters must now be put to work.  Rail lines and bridges need to be taken out and the timing is critical.

Radio communication is too risky.  Information must be exchanged in person.  To that end, a small boat leaves England headed for the French coast.  Two robust young men quietly work the oars.  There�s no moon this night, and the thick overcast is an unexpected yet welcome measure of security.

The third man in the boat is too old and feeble to be of much use rowing.  He works the signal light as they near the shore.  Three quick flashes follow by a pause, and then two flashes.  His counterpart, a retired cavalryman, a veteran of the Great War, is hunkered down in the sand on the French coast.

He signals three quick flashes with his light, then the all clear to come ashore.  The men quickly exchange documents and in an instant they�re gone, the Frenchmen disappearing into the inky shadows.  The three men in the boat swiftly and quietly row back to Mother England.

Now, here�s the interesting part.  The soldiers of the German shore patrol, and there were many of them-- each with the keen eyesight one would expect of young men -- didn't see them or their flashing signal lights.

The question is, why not? I'm going to give you a hint.  It had to do with the old men.

Last week's puzzler:

There was a young gal who had a few bad accidents. So, her grandmother decided to give her an unusual gift: a large sum of money with which she was to buy a brand-new Volvo -- a nice, safe car.

There was one condition, however. When she got the car, grandma wanted to see it to make sure that she didn�t take the money and go out and buy a Firebird.

So, on the first available Saturday she decides to drive to grandma�s house, which is 120 miles away. Because she�s not particularly eager to get there, she gets on the highway and sets the cruise control for 40 miles an hour.

She drives 120 miles to grandma�s house. Her new car has a little computer that tells her that her average speed is 40 miles an hour.

She gets there, shows grandma the car and leaves. On the way back, she�s eager to get back home because she wants to get to the tattoo parlor before it closes.  She sets the cruise control for 60 miles an hour.

She travels the same road and the same 120 miles. When she gets home, she does a little figuring. She says, "I drove 120 miles up, 120 miles back, or 240 miles. I drove 40 miles an hour up, and 60 miles an hour back, so my average speed was 50 miles an hour, and it should have taken me 4.8 hours.

"But it took me 5 hours!"

How can that be?

Last week's puzzler answer:

Well, she didn't calculate the average speed; she calculated the average of the speeds. In fact, her average speed is not 50 miles an hour but it's really 48 miles an hour. And you arrive at that by calculating the total distance divided by the total time. The total distance being 240 miles, and the total time we already know. So it's 240 divided by five hours. Which comes out to be 48 miles an hour.

_______________________
Scott MacLean
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