Question

A fourth grade teacher gives homework every night in both mathematics and language arts. The time to complete the mathematics homework has a mean of 10 minutes and a standard deviation of 3 minutes. The time to complete the language arts assignment has a mean of 12 minutes and a standard deviation of 4 minutes. The time to complete the mathematics and the time to complete the language arts homework has a correlation rho = -0.75. The variance of the time to complete the entire homework assignment is

Answer 1

4

Explanation:

Given that:

X = time taken to complete mathematics homework

Y= time taken to complete language homework

Standard deviation of X ; SD(X) = 3

Standard deviation of Y ; SD(Y) = 4

rho = - 0.375

rho = Cov(X, Y) / SD(X) * SD(Y)

-0.375 = COV(X, Y) / 3 * 4

COV(X, Y) = 12 * - 0.375

COV(X,Y) = - 4.5

Variance(X, Y) = V(X, Y)

V(X, Y) = V(X) + V(Y) + 2COV(X,Y)

V(X) =SD(X)² = 3² = 9

V(Y) = SD(Y)² = 4² = 16

V(X,Y) = 9 + 16 + 2(-4.5)

V(X, Y) = 16

SD(X, Y) = sqrt(V(X, Y)) = sqrt(16) = 4

Hence, standard deviation to complete entire homework, SD(X, Y) = 4