Simple stats problem : Unable to get final answer

I have a simple stats problem where the following table is given.

Speed (Km/h)--------Frequency-----------Cumulative Frequency

0<s<=40--------------14----------------------14

40<s<=60-------------51----------------------65

60<s<=70-------------157---------------------222

70<s<=80-------------141---------------------363

80<s<=100------------37----------------------400

I am asked to find the median which i can find easily 51, but the standard deviation i tried all sorts of values but unable to get the given answer which is 64.6. I am getting like 13.67. Can someone guide me out on this issue ?