Standard deviation

[anne211981]

Hi sorry first of all if i have placed this in the worng place. Having major trouble working my answer out as my tutor has told me its worng but i cant see how its wrong.
First of all my mean is 17.65
my total for sigma fx2 (fx squared) is 33193.

now i was given the equation of fx2-mean/ 100 square rooted.

so i have place in 33193 - 17.65 squared divided by 100 = 32.88.5 / 100 squared rooted = 18.13

this is wrong apprently. but where have i gone wrong.

table of figuers is
number of day (x) number of days squared(x2) number of seeds (frequency) Frequency x days Frequency x days squared
10 100 7 70 700
13 169 18 234 3042
16 256 29 464 7424
19 361 22 418 7942
22 484 11 242 5324
25 625 9 225 5325
28 784 4 112 3136
= 2779 = 100 = 1765 = 33193

all these figure i have been told are correct. with the mean being 17.65
Please someone help me before i go mad.....