You simply have to count the odds in your favour with each choice, and divide them by the number of all possible cases. Since "SUMMER VACATION" has 14 letters in total, and only one of them is a C, you have 1 chance in 14 of picking a C, or if you prefer
[tex] P(\text{Picking a letter C}) = \cfrac{1}{14} [/tex]
Similarly, 2 of the 14 letters are M, so this time your chances are a bit higher:
[tex] P(\text{Picking a letter M}) = \cfrac{2}{14} = \cfrac{1}{7} [/tex]
