Independent solution

How to solve this Covariance and Correlation question

Setup

Setup

Represent the equal marginal variances by v and the unknown covariance by c.

v=Var(X)=Var(Y),c=Cov(X,Y)v=\operatorname{Var}(X)=\operatorname{Var}(Y),\qquad c=\operatorname{Cov}(X,Y)

Model

Model

Expand both stated linear-combination variances, retaining the covariance cross terms.

2v+2c=10v+c=52v+2c=10\quad\Longrightarrow\quad v+c=5
v+4v4c=165v4c=16v+4v-4c=16\quad\Longrightarrow\quad 5v-4c=16

Compute

Compute

Substitute v=5-c into the second equation.

5(5c)4c=165(5-c)-4c=16
259c=16c=125-9c=16\quad\Longrightarrow\quad c=1

Answer

Answer

The covariance is 1.0.

Cov(X,Y)=1.0(D)\boxed{\operatorname{Cov}(X,Y)=1.0\quad\text{(D)}}