To answer this question we should see how much work can be done per day by both workers. To figure this out we write out
3/5 of work done in 9 days
and divide both by 9 to get the amount of work per day which gives us (I will leave the answers in fraction form for the most precise answer)
1/15 of work done in 1 day
Now since this is the work done by two people we can assume that with only one person working the percentage of work done will reduce by half so we can say
1/30 of work by one man in 1 day
Now that we know that we must find out the total amount of work load which is known to be
3/5 + 1/30*4 = 11/15
so now we divide the workload by the amount of daily work produced to figure out how many days it will take paul to finish it so
11/15 divided by 1/30 = 22
The answer to this problem is 22 days
