Question 1094216
r, work rate for 1 tractor
x, number of days using 3 tractors  ("one more tractor")


{{{system(2r*10=1,2r*4+3r*x=1)}}}


{{{r=1/20}}}
-
{{{(8+3x)(1/20)=1}}}

{{{3x+8=20}}}

{{{3x=12}}}

{{{x=4}}}



4 days with 2 tractors and then 4 days with 3 tractors;
total of 8 days.