i dont understand how to compute the beta for question b

Mr. Geller collected information regarding the following stocks and portfolio: Portfolio P1 Security Security B E(T) 5 % 10% Standard Deviation Weight Security A 40% Weight Security B 40% Weight Security C 20% Security C 3% 0% 20% Mr. Geller also has information regarding the following variance covariance matrix: Variance - Covariance Security A Security B Market Portfolio Security A 0.0064 Security B -0.005 Market Portfolio 0.05 0.025 0.04 004 Assume that the CAPM model is valid. Consider additionally that the expected return from the Market Portfolio is 18%. a) (1 point) Compute the expected return and volatility (standard deviation) for portfolio Pi. Solution: E() = 0.4x0.05 +0.4x0.1 +0.2x0.03 = 6.60% Var[e(p)] = 0.42x0.0064 +0.4x0.04 + 2x0.4x0.4X(-0.005) - 0.005824 Op = V0.005824 = 7.63% b) (1.5 points) Compute the expected return, volatility (standard deviation) and beta of a portfolio invested 20% in security A and 80% in portfolio Pl. Solution: The portfolio mentioned is equivalent to: WA = 52%, wg = 32% and we = 16% Erp) = 0.52x0.05 + 0.32x0.1 +0.16x0.03 6.28% Var[E(r)] = 0.522 0.0064 +0.322x0.04 + 2x0.52x0.32X(-0.005) = Op. = V0.005824 = 6.45% 0.05 PA2 = 1.25 0.025 PB02-0.625 Be=0 => W =0.52x1.25 +0.32x0.625 +0.16x0 =0.85