Let
denote the number of packages brought by customer
. If, for instance,
customers are in line, then the total number of packages is
, and

since the number of packages brought by any customer is independent from the number of packages brought by any other.
a.
is the probability that a total of 3 packages are waiting to get wrapped from among a total of 3 customers. This means each customer must have exactly one package. So

b.
is the probability that a total of 11 packages are brought by 4 customers. The only way for there to be 11 packages is if 3 customers bring 3 packages and the last 1 brings 2. There are
ways this can happen, so

2m -16 + 5m + 45 = 7m + 29
The ANWSER is unrestricted domain
I think the answer here would be D. When it rains several inches, the water level of the lake increases, because the word Causation means: the relationship between cause and effect; causality. If it rains a lot, it will cause the lake to increase.