lets

m = the days 1 man needs to finish the job thus 1 man has a rate 1/m jobs per day

w = the days 1 woman needs to finish the job thus 1 woman has a rate 1/w jobs per day

8 men or 12 women can do a piece of work in 25 days

3/m = 1/25 => m = 3*25 = 75 days

5/w = 1/25 => w = 5*25 = 125 days

the together rate of 6 men and 5 women is

6/75 + 5/125= 3/25 = 1/(25/3) = 1/8.33

if 6 men and 11 women work together the work will be done in 8.33 days.