In a hydrogen spectum if electron moves from `7` to `1` orbit by transition in multi steps then find out the total number of lines in the spectum.

Explanation

`{:(,"Lyman",=,(n_(1)-1)=7-1=6),(,"Balmer",=,(n_(2)_2)=7-2=5),(,"Paschen",=,(n_(2)-3)=7-3=4),(,"Bracket",=,(n_(2)-4)=7-4=3),(,"Pfund",=,(n_(2)-5)=7-5=2),(,"Humphrey",=,(n_(2)-6)=7-6=1),(,"Total",=,21):}`
Total number of lines can be calculatd as follows :
Total number of lines `= ((n_(2)-n_(1))[(n_(2)-n_(1))+1])/(2) = ((7-1)(6+1))/(2)=(42)/(2)=21`