Question 777773
A can do a piece of work in 12 days
A one day work is =1/12
B can do a piece of work in 18 days
B one day work  is =1/18
A and B both together work  for one day=  1/12+1/18
                                       =5/36 part 
let total work completed in x days 
both work for (x-3) and B work alone  3 days 

both work for (x-3) days =(x-3)5/36=5x-15/36
B alone three days work =3x1/18=1/6

as per give conditions 
1/6+ 5x-15/36 =1
by LCM  solving  we will get 
-9+5x/36=1
-9+5x=36
5x =45 
x=45/5=9 days
answer work completed in  9 days