Answer :
C. 60
To find the variance of the sum of three independent random variables, you can use the formula for the variance of the sum of independent random variables.
Var(X1 + X2 + X3) = Var(X1) + Var(X2) + Var(X3)
The variances of X1, X2, and X3 are given as sigma1^2, sigma2^2, and sigma3^2 respectively, so the variance of X1 + X2 + X3 is
Var(X1 + X2 + X3) = Sigma 1^2 + Sigma 2^2 + Sigma 3^2
= 20 + 20 + 20
= 60
To find the variance of X1 + X2 - X3, you can use the formula for the variance of differences in independent random variables.
Var(X1 + X2 - X3) = Var(X1) + Var(X2) + Var(X3)
= Sigma 1^2 + Sigma 2^2 + Sigma 3^2
= 20 + 20 + 20
= 60
So the answer is C) 60.
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