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 on of meters. If the standard devastation of the 10 running times is 27.4 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?
Choose the correct choice from the following options
A) one	B) two	C) three	D) four	E) five
The most time consuming part in this question is to define the mean. Under exam pressure and time pressure it is very easy to make mistake.   it is easier to group numbers:  130*3= 
<gadget id="calculator"> 130*3</gadget>
<output>390</output>
390; 105*2=210; 75+85=160; 70+80=150; 90;   Next stage combine results, again using more convenient ways to calculate: 390+210= 
<gadget id="calculator">390+210</gadget>
<output>600</output>
600; 160+150=310; 90. 600+310+90=1000. Since there are 10 numbers the mean is 100. Questions asks to find the quantity of numbers one SD BELOW the mean, which is 100-27,4=72,6. There are only two numbers below 72,6. The answer is two
<result>B</result>
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 on of meters. If the standard devastation of the 10 running times is 27.4 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?