Enigma 1452 - Crossed lines
- From: Chappy <petergregorychapman@xxxxxxxxxxx>
- Date: Mon, 03 Sep 2007 22:35:35 -0700
Enigma 1452 - Crossed lines
New Scientist magazine, 21 July 2007.
by Susan Denham.
+---+---+---+---+
| | | | | How many rectangles
+---+---+---+---+ can be seen in this
| | | | | 4-by-4 grid (show
+---+---+---+---+ on the left?
| | | | |
+---+---+---+---+ In fact there are
| | | | | exactly 100.
+---+---+---+---+
I made a much larger square grid with lines
dividing it into little squares (a three-
figure number of little squares, in fact) and
I calculated the number of rectangles which
could be seen in this new grid. I then cut
the grid along one of the lines in order to
make two rectangular pieces and I calculated
the number of rectangles visible in each of
my two new pieces. The total of these two
numbers was exactly two-thirds of the number
visible in my original large square grid.
What was the size of my original grid before
I cut it?
Ciao,
Chappy.
.
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