There are n coins, each showing either heads or tails. We would like all the coins to show the same face. What is the minimum number of coins that must be reserved?

Its nCr = n! / (r!. (n-r) ! = n!/!. (n-1) ! = n

Explanation:

Here. the arrangement of outcome does not matters, and hence it is a combination. You should know that when arrangement matters then we have to calculate the permutation and its formula is n! / (n-r) !. However, here it is the combination, and hence its nCr or n!/r!. (n-r) !. And here r is 1 as we can have all heads or all tails. And n is n. Hence,

Its nCr = n! / (r!. (n-r) !

=n!/!. (n-1) !

= n