Probability using Permutatins and Combinations

Permutation-set of items selected and arranged-order matters
Combaination-set of items selected-order does not matter

Here have some examples for Probability using Permutations and Combinations:

Example 1) Aclass consists of 10 men and 8 women. Four members are to be Selected( mean use Combination to calculate this question) at random to represent the class. What is the probability that the selection will consist of two men and two women?
P(two men and two women)
=10C2x8C2/18C4
=1260/3060
=0.14

Example 2) If the letters in the word CALCULUS are arranged ( Premutation) what is the probability that...
a) the C's are together
P(C's are together)
=(7! 2!/2!2!2!)/(8!/2!2!2!)
=1/4

b) the C's are apart
P(C's are apart)
=1-1/4
=3/4