Question 765058
THREE MAIN PARTS OF THE FIELD-PLOWING.


FATHER FOR 3 DAYS


Rate is {{{1/8}}} job per day.  We can find how much job was done and what still needs to be done.

{{{(1/8)*3}}} job was done.
{{{1-(1/8)*3}}} job still needs to be done.
That is {{{highlight(5/8)}}} job still needs to be done.


FATHER & SON FINISH


They plow the field in 3 days, suggesting they finished the field.  We do not know their combined rate nor the son's but we can use a variable for the son's rate. 

{{{(1/8+r)*3=5/8}}} using r for the son's rate of work, allows us to find the son's rate if he were to work alone.
Solve for r.

{{{3r+3/8=5/8}}}
{{{3r=5/8-3/8}}}
{{{3r=1/4}}}
{{{highlight(r=1/12)}}} which means, the son could plow the field alone in 12 days.


SON ALONE TO FINISH

Right!  Already answered.  The rate for the son is 1/12 job per day or as the reciprocal, 12 days to plow the field himself.