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

The maximum number of emission lines when the excited electron of a H atom in n = 6 drops to the ground state is $\frac{(n_2-n_1)(n_2-n_1+1)}{2}=\frac{5\times 6}{2}=15$ lines.