46 . A
You can find the mean by adding these terms and dividing the sum by four, the number of
terms in the set.
Now divide this sum by 4:
47 . D
You can start by counting the number of ways this outcome can occur. There are
ways that exactly two of four letters will arrive in two days or sooner (the first and second
letters, the first and third, the second and fourth, etc.).
The probability of a letter failing to arrive in two days or sooner is
So the probability of the first two letters arriving in two days or sooner, and the other two
arriving later than that is:
The probability of any other pair of letters arriving in two days or sooner and the other two
arriving later than that is the same. The probability of any of those outcomes occurring is the
sum of all of them. Since they are the same, the probability of exactly two arriving in two
days or sooner is
48 . B