Question: 2014 Winter Midterm - 15
We choose a bitstring of length 25 uniformly at random. What is the probability that this string
contains at least two 1s?
(a)
$1 - (1/2)^{25} - 25(1/2)^{25}$
(b)
none of the above
(c)
$\sum_{k=2}^{25} {{25}\choose{k}}(1/2)^{k}$
(d)
$1 + (1/2)^{25} - 25(1/2)^{25}$