还是问PREP数学，已解决

redo

D=No. of directors, M=No. of managers, Ad=the average salaries of directors, Am=the average salaries of managers, ask for D/(D+M).
Ad-Am=20000.  Ad-15000=(Ad*D+Am*M)/(D+M)=Ad*D/(D+M)+Am*M/(D+M)=Ad*D/(D+M)+(Ad-20000)*(1-D/(D+M)=Ad*D/(D+M)+Ad-Ad*D/(D+M)-20000+20000*D/(D+M)=Ad-20000+20000*D/(D+M).  D/(D+M)=5000/20000=1/4
考试时，碰见这种题，认倒霉吧