Question 1201606
{{{N*R*T=J}}}
N workers
R rate of jobs for any one worker
T time
J number of jobs


---------------------------------------------------
using 2 tractors a farmer can dig a farm in 20 days,
----------------------------------------------------


{{{2*r*20=1}}}

{{{r=1/40}}}


-------------------------------------------------------------------------------------
he uses the 2 tractors for the first 8 days and added 6 more tractors to complete the work.
------------------------------------------------------------------------------------------


Complete the Work means, finish one whole job.
First eight days, 2 tractors.  Remaining time, 2+6=8 tractors.


{{{2*r*8+8*r*x=1}}}

{{{(2*8)(1/40)+8(1/40)x=1}}}

{{{16+8x=40}}}

{{{2+x=5}}}

{{{highlight(x=3)}}}---------Three days to COMPLETE the work with the eight tractors,  after the first three days with the two tractors.