What's the difference between percent abundance and relative abundance?

Relative means you take the required amount from the actual amount which gives you the abundance and its relative because you compare it to the required amount. 
The percent abundance is the relative abundance as a percentage of the required amount. 
for example: % abundance= (abundance/required amount) X 100 

Determine the  wavelength of light emitted when an electron  in an li2+ ion  makes a transition from an orbital with n  =  6 to an orbital with n  = 5.