A carpenter and his assistant can do a piece of work in 3 days. If the carpenter himself could do thework alone in 5 days, how long would the assistant take to do the work alone?

  7 1/2 days

Step-by-step explanation:

The carpenter can do 1/5 of the work in 1 day. The assistant, who takes 'a' days to do the work, can do 1/a of the work in 1 day. Together, they can do 1/3 of the work in 1 day:

  1/5 + 1/a = 1/3

  1/a = 1/3 -1/5 = 2/15

  a = 1/(2/15) = 15/2 = 7.5

The assistant takes 7.5 days to do the work alone.