The variables [math]S[/math] and [math]T[/math] have the joint density function

[[math]] f(s,t) = \begin{cases} \frac{2}{675}, \quad 0 \lt s \lt 15, 0 \lt t \lt 3s \\ 0, \quad \textrm{otherwise} \end{cases} [[/math]]

Determine [math]\operatorname{Cov}(T,S)[/math].

• -56.25
• 0
• 18.75
• 68.75
• 168.75