Two teams, working jointly, complete harvesting a plot in 4

First of all notice that A, B and C just cannot be correct. The time needed for any of the teams to complete the job working alone must be more than the time needed for both of them to complete the job working together. Thus, the time needed for any of the teams to complete the job alone must be more than 4. So, the answer is either D or E.

If one team need 5 and another 11 days to complete the job, then their combined rate is 1/5+1/11=16/55 job/day, but since we are told that working together they can do the job in 4 days, then their combined rate must be 1/4 job/day. So, D is not correct. Only E is left.

Answer: E.

Algebraic approach:

Say one team needs x and another x+6 days to complete the job separately.

The rate of the first team would be 1/x job/day and the rate of another would be 1/(x+6) job/day, thus their combined rate would be 1/x+1/(x+6) job/day.

We are told that working together they can do the job in 4 days, so their combined rate is 1/4 job/day.

Therefore, we have that 1/x+1/(x+6)=1/4. At this point it's better to substitute the values for x. Answer choice E works, if x=6, then 1/6+1/(6+6)=1/4.

Answer: E.

Hope it helps.