for buffer solution
pH = pKa + log [ salt ] / [Acid] …..(1)
where Ka = Dissociation constant of acid
in this question pH = 6.04 , pKa = 4.74 , [ salt ] / [Acid] = ?
from eq. (1)
6.04 = 4.74 + log [ salt ] / [Acid]
1.3 = log [ salt ] / [Acid]
[ salt ] / [Acid] = Antilog (1.3)
[ salt ] / [Acid] = 1.995 * 10 = 19.95