From: radams@BNR.CA (Rodney Adams)
Subject: Re: A Short Puzzle
Reply-To: "Rodney Adams" <RADAMS@BNR.CA>
Organization: Bell-Northern Research, Ltd.
Distribution: na
Date: Fri, 04 Sep 1992 17:02:23 GMT

In article <1992Sep4.055131.26719@gn.ecn.purdue.edu>, mechalas@gn.ecn.purdue.edu (John P. Mechalas) writes:
|> In article <1992Sep4.050444.27407@unixg.ubc.ca> Sundar_Prasad@mtsg.ubc.ca writes:
|> >In article <1992Sep3.010437.28296@gn.ecn.purdue.edu> mechalas@gn.ecn.purdue.edu
|> >(John P. Mechalas) writes:
|> >>Here's an interesting puzzle someone pulled on me today:
|> >>
|> >>   Imagine a frog that is in square three of a row of six squares.  Square   
|> >>five contains food that the frog wishes to eat.  The frog can only move by
|> >>jumping three squares one direction, then stopping and jumping two squares
|> >>in the other direction.  Is it possible for the frog to land on the square
|> >>with the food without jumping off the "playing field"?
|> >>
|> >>     -------------------------------------------------
|> >>     |   1   |   2   |   3   |   4   |   5   |   6   |
|> >>     |       |       | Frog  |       | Food  |       |
|> >>     -------------------------------------------------
|> >>
|> >If I'm not missing any tricks, the only possible solution is if the frog
|> >can START with a 2 square jump (the above puzzle only says alternate jumps
|> >of 3 and 2 squares, but not that the first jump HAS to be a 3 square one).
|> >Am I right ;-) ?
|> 
|> You are on the right thread, though not quite right.  This puzzle was created
|> by someone (whose name I can't remember offhand) to show how people commonly
|> assume information about a problem that is not actually given.  In this case,
|> it is assumed that the frog is starting in square three.  But, the problem
|> never states whether or not the frog is *starting* to move, or if it is in
|> the *middle* of a move.  It only says that the frog is currently in square 3.
|>    Hence, if the frog started from square six, it jumped three squares to
|> it's current position, and is currently stopped there.  Now, it finishes
|> it's move by jumping the next two squares to the food.

Or, you can do it this way:

>    Imagine a frog that is in square three of a row of six squares.  

OK, I'm imagining a frog in square three, as depicted below.  Meanwhile, 
the REAL frog is sitting in square six.  He then hops to square three and
back to square five.
                                                                     Square   
> five contains food that the frog wishes to eat.  The frog can only move by
> jumping three squares one direction, then stopping and jumping two squares
> in the other direction.  Is it possible for the frog to land on the square
> with the food without jumping off the "playing field"?
>
>      -------------------------------------------------
>      |   1   |   2   |   3   |   4   |   5   |   6   |
>      |       |       | Frog  |       | Food  |       |
>      -------------------------------------------------

I could also have imagined the frog in square three, while the REAL frog
started in square four.  If the playing field is wrapped around a curved
surface so that squares four and six are adjacent, the frog can the hop 
to square one and back to square five.  Of course, this would require some
rather convoluted means to allow the frog to jump in this manner, since
squares four and one would lie on opposite sides of the object.


----------------------------------------------------------------------

Rodney G. Adams
The opinions expressed above are not necessarily those of my employer.
As if you didn't already know that....
"A man must have a certain amount of intelligent ignorance to get 
anywhere."                                      - Charles F. Kettering
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