work and time
posted by suraj .
If 15 men or 24 women or 36 boys can do a piece of work in 12 days, working 8 hours a day, how many men must be associated with 12 women and 6 boys to do another piece of work 9/4 times as great in 30 days working 6 hours a day?

Convert the rates to work/mandays or work/womandays, etc.
So rate1=job/12*15*8 manhours
rate2=job/12*24*8 femalehours
rate2=job/12*36*8 boyhours.
Finally, the equation is
9/4 job = Xmen*6hrs/day*30days*rate1+ 12women*30days*6hrs/day*rate2 + 6 boys*30days*6hrs/day *rate3
solve for Xmen.