[plu]

One Spring day the atmospheric temperature and pressure are 15.0 ^oC and 101.23 kPa respectively, and the air contains 2% by volume of water vapour.  The next day the temperature is still 15.0 ^oC but the pressure has dropped to 100.47 kPa.  Given that the average relative molecular weight of the gases in dry air is 28.94, the water vapour content of the air is now:
A. 0%   B. 1%   C. 3%   D. 4%   E. 5%

This question has been bugging me for a while now.  I simply don't understand why the water vapour content in the air would change with a change in pressure.  Wouldn't all componenets of the air compress proportionally, thus keeping the water vapour content constant?  Thanks for any help, mates  

[ozonoyo]

Look for wet bulb temperature and dry bulb temperature in some book. Probably, you will found some interesting concepts to solve your problem.
I don´t remember now the name of a good book, but some thermodinamics book, or some one about air humidification will include this topic.