10 presents can be ordered in 10! different ways;
presents can then be grouped 1-2, 3-4, 5-6 and so on
however, "double counting" happens in two ways -
one ... in each group of 2 there is double counting by a factor of 2! .
two ... the groups are double counted in 5! different ways.
the answer = ((10!)/(((2!)^5)*(5!)))
in general ...
for n items
and k items per group
where n=km
the formula is:
((n!)/(((k!)^m)*(m!)))
... i think? :(
