Question

In five rolls of two fair dice, what is the probability of obtaining a sum of 9 exactly twice?

Answer 1

9/36 perhaps....

Answer 2

the probability of obtaining a sum of 9 once is 4/36 or 1/9
then use the formula from before : P= (n!/[k!(n-k)!] ) p^k * q^(n-k)
hence the probability of obtaining a sum of 9 twice in 5 rolls = 5C2 * (1/9)^2 * (8/9)^3