you can also use the following.
Let M = no of managers
Let D = no of directors
Let AE = Average salary of all employees
Avg sal of all directors = AE + 15000
Avg sal of all managers = AE - 5000
Now equate sum of salary of all directors = salary of all employees - salary of all managers.
D*(AE + 15000)= (D + M)*AE - M*(AE - 5000)
-->D*AE + 15000*D = D*AE + M*AE - M*AE + 5000M
--> D/M = 5000/15000 = 1/3
--> D/(D+M) = 1/4
Hence answer is C. I need not explain why it can't be A or B.