Just a little more help on conduction

Ok I solved the last one past exam question by myself but this one has been bugging me for days, any help would be appreciated.

A composit wall consists of three materials a, b and c, two of which are of known thermal conductivity ka=20W/mK and kc=50W/mK and known thickness La=0.3m and Lc=0.15m. The third material which is sandwiched inbetween a and c is of known thickness Lb=0.15m but unknown thermal conductivity, kb. Under steady state conditions, Ts,o(temp on surface of material c)=20°C and Ts,i(temp on surface of material a)=600°C. The temp inside T∞=800°C. The inside convection coefficient h=25W/m^2K

a What is the value of kb

b What are the temperatures at the interfaces of a & b and b & c

I thought about using the thermal resistance equations to solve it but i cant seem to get arrange an equation for it.

Cheers TNix

Merged post follows:

Ok I have again figured out my own answer. It was invloving the resistance equations to get q''(flux)=(T2-T1)/((La/ka)+(Lb/kb)+(Lc/kc)) then using the flux equation q''=h(deltaT) then rearranging the first to get kb. The final answer is 1.53W/mK