Solution: 2019 Fall Final - 3

Author: Michiel Smid

Question

Consider strings of length 85, in which each character is one of the letters $a,b,c,d$.
How many such strings have exactly 15 letters $a$ or exactly 30 letters $d$?

(a)

None of the above.

(b)

${85 \choose 15} \cdot 3^{70} + {85 \choose 30} \cdot 3^{55}$

(c)

${85 \choose 15} \cdot 4^{70} + {85 \choose 30} \cdot 4^{55}$

(d)

${85 \choose 15} \cdot 3^{70} + {85 \choose 30} \cdot 3^{55} - {85 \choose 15} \cdot {70 \choose 30} \cdot 2^{40}$

Solution

Let’s break me down

Let A be the set of all strings that have exactly 15 letters a
We choose 15 of the 85 positions to be a: $ \binom{85}{15} $
The remaining 70 positions can be any of the 3 letters: $ 3^{70} $
Thus, the total number of strings is $ \binom{85}{15} \cdot 3^{70} $
Let B be the set of all strings that have exactly 30 letters d
We choose 30 of the 85 positions to be d: $ \binom{85}{30} $
The remaining 55 positions can be any of the 3 letters: $ 3^{55} $
Thus, the total number of strings is $ \binom{85}{30} \cdot 3^{55} $
Let's determine $A \cap B$
We choose 15 of the 85 positions to be a: $ \binom{85}{15} $
We choose 30 of the 70 remaining positions to be d: $ \binom{70}{30} $
The remaining 40 positions can be any of the 2 letters: $ 2^{40} $
Thus, the total number of strings is $ \binom{85}{15} \cdot \binom{70}{30} \cdot 2^{40} $

Now, we can determine $A \cup B$

$ |A \cup B| = |A| + |B| - |A \cap B| $

$ |A \cup B| = \binom{85}{15} \cdot 3^{70} + \binom{85}{30} \cdot 3^{55} - \binom{85}{15} \cdot \binom{70}{30} \cdot 2^{40} $