What is the maximum number of emission lines when the excited electron of a $H-atom$ in $n=6$ drops to the ground state?

When the excited electron of an H atom in n = 6 drops to the ground state, the following transitions are possible:

The number of spectral lines: $\frac{n(n-1)}{2}$

where $n=6$

Hence the total number of spectral line will be: $\frac{6\times 5}{2}=15$