Answer:
60% of your complete portfolio should be invested in the risky portfolio if you want your complete portfolio to have a standard deviation of 12%.
Explanation:
Let the % of investment in Treasury Bill be X. It is important to note that treasury bill is a risk free investment, hence standard deviation would be 0.
% invested in Risky Asset = 1-X
Expected Standard Deviation = X*Standard Deviation of X + (1-X)*Standard Deviation of Risky Asset
12% = X*0 + (1-X)*20%
12% = 20% - 20%X
X = (20% - 12%)/20% = 40%
% Investment in Risky Asset = 1-40% = 60%
Answer is 60%