Bigger animals tend to carry their young longer b/4 birth. The length of horse preg. from conception to birth?

Question

varies according to a roughly normal distribution with mean 336 days and std. deviation 3 days. Use the 68-95-99.7 rule to answer the following.
a. What percent of horse pregnancies are longer than 339 days?.
b. Almost all (99.7%) horse pregnancies fall in what range of length ?.

Answer 1

1 std deviation is 336 +/- 3
2 std deviation is 336 +/-6
3 std deviation is 336 +/-9
So 339 falls at the end of 1 std deviation which means that 68 % are less than or equal to 339 days, you figure out the rest. And at 99.7% or 3 std dev., pregnancies are between (336-9)(336 +9) days