Answer:
Standard deviation = 7.59
Explanation:
Standard deviation is the sum of the  squared deviation of the individual return from the mean return under different scenarios
The mean  return = ∑R×P
r- return under circumstance, Â P- probability
Mean return =(0.3×20%) + (0.4×10%) + (0.3×5%)
    = 11.5%
Outcome  R     (R- r )^2           P×(R- r )^2
Strong   20   0.3×(20-11.5)^2       21.67    Â
Normal    10    0.4×(10-11.5)^2        23.4
Weak      5    0.3×(5%-11.5)^2       2.67
                                 57.75
Standard deviation =  √57.75 = 7.59