Several folks have asked what is the use of the valve timing diagram. Its only use is to show, in graphic form, the sequence of events that happen in the combustion chamber, and in particular, when the valves open and close relative to the crank angle. It is simply a visual tool; you could do the entire thing with words, but it would be more difficult to understand.
Use of valve timing is to enhance the efficacy of engine,by allowing fresh air in compression stroke sothat more power will generate ,exhaust valve open earlier sothat less work done is required to push it out and etc.
That question has no general answer. The contact ratio for a gear pair depends upon the particular gears and the way they are mounted (it cannot be determined for an unmounted pair).
Tell your "question paper" that it is nuts, that the question is ill posed, and shame on it!