Equivalent thermal conductivity

A small cylinder of radius $r$‍ is placed inside a big shell having inner radius $r$‍ and external radius $4r$‍. The ends of the cylinders are maintained at different temperatures as represented below.

The conductivity of the inner cylinder is $5\text{K}$‍ and that of the outer cylindrical shell is $2\text{K}$‍.

$k_{\text{eff.}}=$‍ $\text{K}$‍