Question: 2015 Fall Final - 3
How many bitstrings $s_1s_2 \dots s_{20}$ of length 20 have the property that $s_1s_2s_3 = 000$ or
$s_2s_3s_4 = 000$?
(a)
$2^{17} - 2^{16}$
(b)
$2^{18} - 2^{16}$
(c)
$2^{17} - 2^{15}$
(d)
$2^{18} - 2^{17}$