Final answer:
The range in which we would expect to find the middle 68% of most averages for the lengths of pregnancies in a sample of size 31 is the mean +/- one standard deviation.
Step-by-step explanation:
The question is asking about the range in which we would expect to find the middle 68% of most averages for the lengths of pregnancies in a sample of size 31. To solve this, we can use the concept of confidence intervals. The middle 68% of averages falls within one standard deviation above and below the mean.
In this case, since the sample size is 31, we can use the formula: standard deviation of sample mean = population standard deviation / square root of sample size. So, the range in which we would expect to find the middle 68% of most averages for the lengths of pregnancies in a sample of size 31 would be the mean +/- one standard deviation.