Math
posted by Debin .
Consider the following test scores obtained by a class.
Test score 90 80 70 60 50
Frequency 1 4 11 6 2
Find the standard deviation (rounded to the nearest hundredth).

I've posted this question earlier and I'm still getting the wrong answer.
I don't know from what point I messed up.....
this is what I did.
Mean:
90+320+770+360+100 / 24 = 68.33
Deviation
469.59+136.19+2.79+69.39+335.99 / 24= 42.25
then square root of 42.45 is 6.5
but my answer came wrong. please help!
I can't move on to my next homework question because I keep getting this wrong :(

Subtract each of the scores from the mean and square each difference. Find the sum of these squares. Divide that by the number of scores to get variance.
For example, with the score of 80:
(8068.33)^2 * 4 = 136.19 * 4 = ?
Do this with all the frequencies.