Actually its not fixed, the weightage of derivations is not disclosed seperately. You have to assume it yourself. OK ill help you out!

Unit................. Marks
1 Electrostatics 08
2 Current Electricity 07
3 Magnetic Effect of Current & Magnetism 08
4 Electromagnetic Induction and Alternating current 08
5 Electromagnetic Waves 03
6 Optics 14
7 Dual Nature of Matter 04
8 Atoms and Nuclei 06
9 Electronic Devices 07
10 Communication Systems 05
Total 70

Now its clear one derivation will surely come from first 2 chapters. One will come from 3 & 4 chapters.. Onw will come from 6 chapter. And one will come from opitcs.

Thank you