I don't quite understand the bypass air I am writing an essay on how a jet engine works, so I need specific answers on this.

What percent of the air that is sucked into the engine is bypass air? I have heard that bypass air provides about 60% of the thrust.  How does cold air provide thrust?  I was under the impression that hot air makes much more thrust than cold.
Cold air accelerated backwards provides forward thrust by reaction. What really matters is the amount of mass accelerated backwards. Its temperature is not important...  

Concerning the 'percent of air sucked', it depends on the design. A pure turbojet doesn't produce 'bypass air', and a turbofan may reach a ratio of 10:1, that ratio being the relation between the mass of 'cold' air pushed backwards by the fan in a given time and the mass of air that passes through the combustion chamber...