I understand how the variance is reached, but how is the STDA and STDB answer calculated?

Calculate the arithmetic mean and sample standard deviation of retums on stocks A & B. Stk A 19S9 1991 (_15+ _00+ _50+ _20)/4 0.1 _50)/4 0.2 VARA [(_15- _1)2+ stcck B Mean Var (RI + + + RT)/T 1 _00833 STDA 9.13% VARB _2)2+ + + _ 2)i/3 _0866 STDB 29.4%