xvxvxv: In question number 1 why I can't use P=V.I
You can, but you have to do it at the normal condition first: so 280=240*Inormal
Inormal = 280/240.
Ohms law says V = IR so 240 = R*280/240 or R = 2402/280
The resistance R, will be approximately the same at reduced voltage, so using Ohm's law again:
200 = Ireduced*2402/280
Ireduced = 200*280/2402=0.972
(PS on question 16, go with thermal conductivity!).