Certify Teacher EC-6 (391) Mathematics (902) Practice Test

The key idea is that one standard deviation around the mean forms an interval from mean minus standard deviation to mean plus standard deviation. With a mean of 60 and a standard deviation of 4, that interval is 60 − 4 to 60 + 4, which is 56 to 64. So values within about one standard deviation lie between 56 and 64. The other options don’t fit because they either aren’t centered on the mean or are narrower/wider than one standard deviation.