Took me 7 min to solve

First: For A to finish this job, it takes him 20 days

Therefor: on one day he can finish \(\frac{1}{20}\) of the job

Second: For B to finish this job, it takes him 30 days.

Therefor: on one day he can finish \(\frac{1}{30}\) of the job

Together they can finish the job in \(\frac{1}{20} + \frac{1}{30}= \frac{1}{12}\) --> 12 Days.

(1): We know B worked for 5 days alone. Question: How much work he has done in 5 days out of the 12 Days?

--> \(\frac{1}{30} * 5 = \frac{5}{30}=\frac{1}{6}=\frac{2}{12}\) (easier to calculate later)

(2): We also know how long A worked with B together. Same question: How much work have they done in 4 days out of 12 days?

--> \(\frac{1}{12}*4 = \frac{4}{12}\)

--> \(1 - \frac{4}{12} - \frac{2}{12} = \frac{6}{12}\)

After 6 Days A left the work.

Maybe not the best approach. And Q29 is harder than this one.