Let resistance of bulb filament be R0 at 0°C using R=R0(1+αΔt) we have R1=R0[1+α×100]=100......(1) R2=R0[1+α×T]=200......(2) On dividing we get
200
100
=
1+αT
1+100α
⇒2=
1+0.005T
1+100×0.005
⇒T=400°C Note : We may use this expression as an approximation because the difference in the answers is appreciable. For accurate results one should use R=R0eαΔT