i can do this on a online calculator, but when im doing it myself its total differnt answer

whast standard deviation for these

10,11,11,12,12,12,13,13,13,13,13,13,14,14,14,14,14 ,14,14,14,15,15,15,15,15,15,15,15,15,15,16,16,16,1 6,16,16,17,17,17,17,17,18,18,18,19,19,19,19,20,20

Ive got mode =15

mean =15.18

median =15

some one help me do the standard deviation pleaee.. know this takes time, ive been doing it last 6 hours none stop and cant do it

thanks