The number of bits needed to encode all letters (i.e., 26),

The number of bits needed to encode all letters (i.e., 26), 10 symbols and all number (i.e.,10) is—

⇒ n = 6, as 2⁶ = 64
if we take n = 5, then 2⁵ = 32 which will not encode 46 number, hence n = 6 taken.