Solution: 2017 Fall Final - 3

Author: Michiel Smid

Question

Consider strings consisting of 40 characters, where each character is one of the letters $a$, $b$, and $c$. Such a string is called cool if

How many cool strings are there?

(a)

$ {{40 \choose 8} \cdot 2^{32} + {40 \choose 7} \cdot 2^{33}}$

(b)

$ {{40 \choose 8} \cdot 2^{32} + {40 \choose 7} \cdot 2^{33} - {40 \choose 15} \cdot {15 \choose 8}} $

(c)

$ {{40 \choose 8} \cdot 2^{32} + {40 \choose 7} \cdot 2^{33} - {40 \choose 15} \cdot {15 \choose 8} \cdot 2^{25}} $

(d)

None of the above.

Let A be the set of strings that contain exactly 8 many $a$'s
We choose 8 positions out of the 40 positions for the $a$'s: $ \binom{40}{8} $
The remaining 32 positions can be filled with either $b$ or $c$: $ 2^{32} $
$ |A| = \binom{40}{8} \cdot 2^{32} $
Let B be the set of strings that contain exactly 7 many $b$'s
We choose 7 positions out of the 40 positions for the $b$'s: $ \binom{40}{7} $
The remaining 33 positions can be filled with either $a$ or $c$: $ 2^{33} $
$ |B| = \binom{40}{7} \cdot 2^{33} $
Let $ A \cap B $ be the set of strings that contain exactly 8 many $a$'s and 7 many $b$'s
We choose 15 positions out of the 40 positions for the $a$'s and $b$'s: $ \binom{40}{15} $
Within those 15 positions, we choose 8 of them to be $a$'s: $ \binom{15}{8} $
$ |A \cap B| = \binom{40}{15} \cdot \binom{15}{8} $

In total, there are $ \binom{40}{8} \cdot 2^{32} + \binom{40}{7} \cdot 2^{33} - \binom{40}{15} \cdot \binom{15}{8} $ cool strings.