Answer: 45%
Explanation:
Standard deviation for the portfolio will be a weighted average of the standard deviations of the individual assets.
Risky asset has standard deviation of 20%. Assume the weight is x.
Treasury bills have a standard deviation of 0 as they have no risk. Assume their weight is y.
Target Standard deviation is 9%.
Formula would be:
9% = (x * 20%) + (y * 0%)
20%x = 9%
x = 9% / 20%
x = 45%