x freq fixi fixi²
20 14 280 5600
50 51 2550 127500
65 157 10205 663325
75 141 10575 793125
90 37 3330 299700
____________________________
400 26940 1889250
what is the standard deviation? i keep getting around 14.. whereas the answer stated 64.6.. can anyone help me pls? tia
and the formula i used is
sqrt (summation(fixi²)-( summation(fixi) / n) / (n-1) )
= sqrt( 1889250 -(26940²/400)/ 399)