Please help if you can. I got the answer to part a) as 47C and 40C but don't know how to do part b. Thanks!

1.53 Consider a surface-mount type transistor on a circuit board whose temperature is maintained at 35C. Air at 20C ows over the upper surface of dimensions 4 mm x 8 mm with a convection coefcient of 50 Wl'm2 - K. Three wire leads, each of cross section 1 mm x 0.25 mm and length 4 mm, conduct heat om the case to the circuit board. The gap between the case and the board is 0.2 mm. Tran sister case Wire lead Circuit board (a) Assuming the case is isothermal and neglecting radi- ation, estimate the case temperature when 150 mW is dissipated by the transistor and (i) stagnant air or (ii) a conductive paste lls the gap. The thermal conductivities of the wire leads, air, and conductive paste are 25, 0.0263, and 0.12 W/m - K, respectively. Using the conductive paste to ll the gap, we wish to determine the extent to which increased heat dis- sipation may be accommodated, subject to the con- straint that the case temperature not exceed 40C. Options include increasing the air speed to achieve a larger convection coefcient k andfor changing the lead wire material to one of larger thermal con- ductivity. Independently considering leads fabri- cated from materials with thermal conductivities of 200 and 400 W/m - K, compute and plot the maxi- mum allowable heat dissipation for variations in I: over the range 50 S h S 250 W/m2 - K