Problem Solving

70, 75, 80, 85, 90, 105, 105, 130, 130, 130
The list shown consists of the times, in seconds, that it took each of 10 schoolchildren to run a distance of 400 meters. If the standard deviation 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 1 standard deviation below the mean of the 10 running times?

A. One
B. Two
C. Three
D. Four
E. Five

Can anybody help me solve this problem please.[/list]

Given S.D = 22.4

You're asked for values < Mean - S.D

Mean = Sum of times/10 = 1000/10 = 100

Mean - S.D = 100 - 22.4 = 77.6

From the list 2 Nos are < 77.6 (70 and 75) B IMO