Mr. Vella can build a brick wall in 4 days. His apprentice can build the same wall in 6 days. After working alone for 3 days, Mr. Vella became ill and left the job for his apprentice to complete. How many days did it take the apprentice to finish the wall?

After working for 3 days, Mr. Vella has finised 3/4 of the work. so for the assistant, 3/4 of 6 is 9/2 which is 4 1/2. So 4 1/2 days of work has been done for the apprentice, so 6 - 4 1/2 = 1 1/2. The apprentice finished the wall in 1 1/2 days.