一道英文统计学题目.求大神指导算Z的数值 成功率

一道英文统计学题目.求大神指导算Z的数值 成功率
Question 3
Because of a severe drought in New South Wales, Farmer Giles needs to move his
cows to another property in Victoria that has not been affected by the drought. It
is known that a group of cows takes, on average, 16 days to make the trip
between the two farms. The time taken to make the trip is normally distributed
with a standard deviation of 2 days.
Farmer Giles has 100 cows in his herd. He can only manage to transport 20 cows
at a time. If it takes him exactly one day to return home after taking each group
of cows, how likely is it that he can transport his herd within 90 days?