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'One Basket or Group Photo' printed from http://nrich.maths.org/
A school photographer is taking a photograph of the two basketball
teams. She has to arrange ten people, all of different heights, in
two rows of five, one behind the other. Each person at the back
must be taller than the person directly in front of them. Along the
rows the heights must increase from left to right.
In how many ways can two, four or six people to be arranged in
this way for a photo, or eight people? In how many ways can the ten
team members be arranged like this for the photo to be taken?
You may even like to generalise the problem to twelve people or
to any specified even number.
Now try the problems
Counting Binary Ops