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

-->15000D=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.

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Regards

Subhen