Question 1131263
<br>
NOTE: "3 times faster than..." means 4 times as fast, as indicated in the solution by tutor @ikleyn.  The other tutor appears to have worked the problem with the faster working 5 times as fast....<br>
Since the faster worker works 4 times as fast as the slower worker, the faster worker does 4/5 of the work when they are working together.  If working together takes them 5 days, then the faster worker does 4/5 of the job in 5 days.  So then the time he needs to do the job alone is 5/4 of 5 days: 5*(5/4) = 25/4 days or 6.25 days.