Answer:
To arrange 7 items, taking only 3 at a time and assuming that the order of those 3 items do not matter, the procedure is as below:
You first pick 3 items out of 7 original items (combination with n = 7 and k = 3), then you pick 3 items out of 4 remaining items (combination with n = 4 and k = 3), the last item is automatically chosen.
=> Number of ways to do this:
N = 7C3 x 4C3
= 7!/(3! x (7-3)!) x 4!/(3! x (4 - 3)!)
= 35 x 4
= 140 ways
Hope this helps!
:)
