Two persons having different productivity of labour, working toger can reap a field in 2 days. If one-third of field was reaped by first man and rest by or one working alternatively took 4 days. How long did it take for faster person to reap whole field working alone?

Total efficiency of two persons = 50% [As they complete work in 2 days. ]
First Person completes work = $$\frac{1}{3}$$ = 33.33% [In 2 days]
Rest work will be completed by Second man = $$\frac{2}{3}$$ = 66.66% [In 2 days]
So, efficiency of second person is greater.
Efficiency of second person = $$\frac{{66.66}}{2}$$  = 33.33% per day
Then, Second person will complete whole work in,
= $$\frac{{100}}{{33.33}}$$  = 3 days.