A and B worked together on a job for 4 hours and finished half of it. A worked thrice as fast as B did. If B then left and A was joined by C and they finished the job in 1 hour, how long would it have taken C to do the whole job alone?

Let us consider B's efficiency to be x units per hour. This makes A's efficiency to be 3x units per hour. Hence, their combined efficiency is 4x units per hour.

In four hours, A and B together will complete 16x units of work, which is half of the total work. By this, the total work is 32x units. 

Now, B left and C joined. A and C together completed the remaining work (which is 16x units) in 1 hour. In this, A must have done 3x units, and C would have done 13x units of work in one hour. Hence, C's efficiency is 13x units per hour.

To complete the total work, C will take $$\frac{32}{13}$$ hours.