Answer:
16%
Step-by-step explanation:
The difference between the time of interest (339 days) and the mean (336 days) is 3 days, which is exactly 1 standard deviation.
The 68-95-99.7 rule tells you that 68% of pregnancies will be within 1 standard deviation. The remaining 32% will be evenly split between pregnancies that are longer than 339 days and ones that are shorter than 333 days. So, half of 32%, or 16%, will be longer than 339 days.