Im working on an introductory finance question and I don't know how to get to the next step. Given: "ABC Inc and XYZ Inc have common shares listed on NYSE. S&P500 represents market portfolio. Portfolios: 1. equal-weighted portfolio of ABC & T-bills has an annual variance of .04 2. equal-weighted port. of S&P500 and T-bills has annual variance of .01 3. equal-weighted port. of ABC and S&P500 has annual variance of .08 Tbills has an expected annual return of 3%, S&P500 has expected annual return of 7% Calculate beta of ABC." ---- from the question I know can assume that betaS&P is 1 and betaTbills is 0, which can give me betaPortfolio2. I can get ReturnPortfolio2 with return(tbills) and return(S&P) the portfolio variance rooted will give me portfolio standard devations, but I still don't know individual variances of any of the 3 assets. I know that betaABC = covariance(ABC,S&P)/variance(S&P) or I can use betaABC = [correlation(ABC,S&P)(std devation ABC)] / std devation S&P but I don't know how to get variance(ABC) or variance(S&P) given only portfolio variance and expected return. Any help is much appreciated