Mean (x + y) = Expected value (X + Y) = E(x) + E(y) which means the mean is meanx + meany, so is meanx = 75 and meany =70, then the mean (x +y) = 75 + 70 = 145

The Var(x + y) = Var(x) + Var(y) if x and y are independent.

Var(x) = 6^2 = 36

Var(y) = 8^2 + 64

Var(x+y) = 100. The standard deviation is the square root of the variance, so standard deviation = 10

Var(x) = 6^2 = 36

Var(y) = 8^2 + 64

Var(x+y) = 100. The standard deviation is the square root of the variance, so standard deviation = 10

The mean of the difference is the difference of the means E(X - Y) = mean (x - y) = meanx - meany

So the difference of the means is 75-70 = 5

The standard deviation of the difference is the square root of the Var(x-y)

Var(x - y) = Var(x) + Var(y) if x and y are independent, same as for Var(x +y) . Therefore the answer is 10

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