Working individually, A would finish a project in 3 days. B takes double the time as taken by A, C takes one more day in addition to the time taken by B. If all three of them decide to work together on the project, how much time would they take to finish it?

A can do the work in 3 days. So, in 1 day a does 1/3 amount of work.

B takes double the time as A. So, B can do the work in 6 days. So, in 1 day B does 1/6 amount of work

C takes one more day in addition to the time taken by B. So, C can do the work in 7 days. So, in 1 day C does 1/7 amount of work

When they work together, in 1 day they complete how much work?
In 1 day A, B and C together complete = 1/3 + 1/6 + 1/7 = 9/14 amount of work
A, B and C together complete the entire work in 14/9 days.