Question 33803
1st belt Data:
  time=12 min    ;   rate = 1/12 job/min
2nd belt Data:
  time=9 min     ;   rate = 1/9 job/min
3rd belt Data:
  time= x min    ;   rate =  1/x job/min
Together Data:
  time= 3 min    ;   rate =  1/3 job/min
EQUATION:
rate + rate + rate= 1/3
1/12  + 1/9  + 1/x  = 1/3
Multiply thru by 36x to get:
3x + 4x + 36 = 12x
5x=36
 x=7.2 minutes  (time for the 3rd belt working alone)
Cheers,
Stan H.