The List consist of the times in seconds that took each of the schoolchildren to run a distance of 400 meters. If the standard deveiation of the 10 running times is 22.4 seconds,rounded to the nearest tenth of a second, how many of the 10 running times are more than one standard deviation below the mean of the 10 running times?

1. One

2. Two

3. Three

4. Four

5. Five

Answer is two, but i dont know how to calcualte this one though it looks simple

... Pleae help me .. have exam in next 3 weeks.

Regards,

Puneet Pareek