Answer:
21.05%
Explanation:
First find the z-score for both 270 and 280 days of pregnancy and their equivalent percentile in the normal distribution.

For X = 270 days:

A z-score of 0.25 corresponds to the 59.871 th percentile
Therefore, 59.871% of pregnancies should last less than 270 days.
For X = 280 days:

A z-score of 0.875 corresponds to the 80.921 th percentile
100 - 80.921 = 19.079
Therefore, 19.079% of pregnancies should last more than 270 days.
The percentage of pregnancies lasting between 270 and 280 days (P) is given by:
P=100 -19.079-59.871
P=21.05%