>>28
The expected number of people who wear glasses in a group of 10 is 0.1 (assuming we have no other information about the group of 10). Consider a similar situation: How many numbers in the set {k, k+1, k+2} (k an integer) are divisible by two? Without knowing k, our answer is 1.5. The reason for this is that either 1 of the numbers must be even, or 2 must be even, and these possibilities occur with equal probability, so the expectance is (0.5)*1 + (0.5)*2. If, on the other hand, we know k is even (or odd) then {k, k+1, k+2} has 2 (1) even numbers.
Also, I'd like to say that it's kind of humorous that you should bring up kindergarten arithmetic, since your mathematics education does not seem to have gone past whole numbers.