calculate thermal conductivity for thermal paste
I think what is confusing is how you get the dimension m (meter). You get that by cross_sectional_area / thickness.

So to get to the more recognizable thermal resistance of defined geometry:

$$ {\theta}_{paste} = \frac{Thick}{Area} \times \frac{1}{K_{paste}} $$

\$ K_{paste} = 1.6 \frac{W}{mK} \$ as given.