See friend
Your question as well as the answer to your question is highly debatable.
It will be very wrong in saying how much of the paper u shud solve etc etc...
It varies according to the toughness of the paper.
sometimes solving 20% of a paper correctly will be considered very great but sometimes it can also be a shame.
So my advice......do not have an idea that "I should solve this much etc"........whatever u mark ..mark it properly.......
AND PLEASE NO GUESS WORK!!!
1 mark can cost u 1000 ranks