Practice By Tag: COMP 2804: Discrete Structures II

Question: 2019 Winter Final - 2

1 . 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 $a$ and exactly 30 letters $d$?

(a)

${85 \choose 15} \cdot {70 \choose 30} \cdot 4^{40}$

(b)

${85 \choose 15} \cdot {70 \choose 30} \cdot 2^{40}$

(c)

${85 \choose 15} \cdot {70 \choose 30} \cdot 3^{40}$

(d)

None of the above.