Some reason I get a different answer than the actual answer.
* = Standard Deviation, which is unknown and that is what we are solving for
$\displaystyle 0.20 = \frac {[0.08 + (0.13 + 0.08)]SD}{0.25}$ ..............multiply both sides by 0.25
$\displaystyle \Rightarrow 0.05 = [0.08 + 0.13 + 0.08]SD$ .............calculate the coefficient of SD
$\displaystyle \Rightarrow 0.05 = 0.29SD$ ...................................Now divide both sides by 0.29
$\displaystyle \Rightarrow SD = \frac {0.05}{0.29} = 0.1724 ...$
ok, so here we can cut out the middle man, and equate the first and last piece, we can do that since all three pieces are equal. i don't know how you ended up with this type of equation to begin with
$\displaystyle 0.20 = 0.08 + 0.2~SD$ ...........subtract 0.08 from both sides
$\displaystyle \Rightarrow 0.12 = 0.2~SD$ ........divide both sides by 0.2
$\displaystyle \Rightarrow SD = \frac {0.12}{0.2} = 0.6$
EDIT: Oh, the last piece is what you simplified the middle piece to get. Ok. Yeah, you still do what I did for the next step