Data Sufficiency

I am taking the test tomorrow. I found this DS question while taking the GMAT from the GMATprep, I do not understand the answer. My answer to the question was (E): not enough information is provided to answer the question. However, the program's correct answer was (C). I do not see it. Would anyone know? Thank you. Here is the question:

Each employee on a certain task force is either a manager or a director. What percent of the employees on the task force are directors?

(1) The average (arithmetic mean) salary of the managers on the task force is $5,000 less than the average salary of all employees on the task force.

(2) The average (arithmetic mean) salary of the directors on the task force is $15,000 greater than the average salary of all the employees on the task force.

It seems to me that more than one combination of total directors salary/# of directors and total managers salary/# of managers exist that meet the two conditions. But, I may be missing something! Thank you.

let a= avg
m= mgrs
d= directors

so a = ((a-5000)*m + (a+15000)*d)/(m+d)

or 5000m =15000d

or d/m = 1/3

d =25%

C

X-avg. salary of M, M-no. of manager and y-avg. salary of D, D-no. of directors.
D/M+D ???
From I X+5= xm+yd/m+d
From II, Y-15= xm+yd/m+d

From I and II, Y-X=20
also solving I gives D/M+D= 5/Y-X= 5/20= .4
ANs is 25 %
Hence C