someone stole my book and im trying to figure outhtis formula can someone?

Question

use the following information to calculate the standard deviatin of Smiths Corp's returns

State------Probability----Return
Boom--------20%----------40%
Normal-------60%---------15%
Recession--20%--------(20%)

can sum1 please help me with a formula for this and how to use the formula--it would mean alot to me--i need this for a grade and some idiot stole my book--please help---i spent all night online trying to figure this out and i cant find any formula for this

Answer 1

http://en.wikipedia.org/wiki/Standard_deviation



Look at this section:



Standard deviation of a discrete random variable or data set