If the first excitation potential of an atom is 20V its first excitation potential will be

Excitaion potential can be calculated using Ionisation potential as:

Ionisation poetential is the poetential calculate from the orbit n=1 to the n = infinity and that is given as:
E = > 13.6*z^2
z^2 = 20/13.6............(1)
Excitation energy is given as : => -13.6*(z^2/4 – z^2/1)
from eqn. (1) we get,
=> -13.6*(5/13.6 – 20/13.6)
=> 15 V