How much faster are airmolecules going when the temperature raise from 15 to 25 degrees C?
User LTK 4 is correct. The air molecule is moving 1.7% faster at 25&deg;C.
Marijn can calculate the absolute velocities from the simple equation given in http://www2.ignatius.edu/faculty/decarlo/surfacepower/pages/KineticTheoryOfGases.htm .

And yes, this is more physics than Earth Sciences.