Question 1206762
<font color=black size=3>
Key assumption: When the workers team up, neither worker slows down another.


The LCM of {12, 20, 18} is 180.
Consider a task of moving 180 boxes. 


A and B need 12 days to do the job. Their combined unit rate is 180/12 = 15 boxes per day.
Formula:  rate = amountDone/time
B working alone needs 18 days. B has unit rate of 180/18 = 10 boxes per day
A working alone has unit rate of 15-10 = 5 boxes per day
C working alone needs 20 days. C has unit rate of 180/20 = 9 boxes per day


Unit rates when working alone
A = 5 boxes per day
B = 10 boxes per day
C = 9 boxes per day


A and C combine their efforts to work at a unit rate of 5+9 = 14 boxes per day.
rate*time = amountDone
Time = amountDone/rate
Time = (180 boxes)/(14 boxes per day)
Time = (180/14) days
Time = <font color=red size=4>12.857 days approximately</font>
Round this value however needed.


Side notes:
180/14 = 90/7
90/7 = 12 & 6/7
</font>