1 = M in the word
4 = I's in the word
4 = s's in the word
2 = p's in the word
1+4+4+2= 11 Letters in total.
The probability of picking an S in the word 'Mississippi', would be
4/11 of picking S in the word Mississippi.
^ 4 = S's in the word.
^ 11 = Letters in total
