Given information:
- A and B can finish a work together in 30 days.
- They worked together for 20 days.
- After 20 days, B left.
- A finished the remaining work alone.

To find:
In how many days A alone can finish the work?

Let's solve the problem step by step:

Step 1: Find the work done by A and B in 20 days
- A and B worked together for 20 days.
- So, the work done by A and B in 20 days = 20 * (1/30) = 2/3

Step 2: Find the remaining work
- The remaining work is equal to the total work minus the work done by A and B in 20 days.
- Total work = 1 (assuming the work as 1 unit)
- Remaining work = 1 - 2/3 = 1/3

Step 3: Find the work done by A in 1 day
- A and B can finish the work together in 30 days.
- So, the work done by A and B in 1 day = 1/30
- The work done by B in 1 day = (1/30) - (work done by A in 1 day)
- Given that B left after 20 days, the work done by B in 20 days = 20 * [(1/30) - (work done by A in 1 day)]

Step 4: Find the work done by A in 20 days
- The work done by A and B in 20 days = 2/3
- The work done by A in 20 days = 2/3 - (work done by B in 20 days)

Step 5: Find the work done by A in 1 day
- The work done by A in 20 days = (2/3) - (work done by B in 20 days)
- The work done by A in 1 day = (work done by A in 20 days) / 20

Step 6: Find the time taken by A to finish the remaining work alone
- The time taken by A to finish the remaining work alone = (remaining work) / (work done by A in 1 day)
- Time taken by A = (1/3) / [(work done by A in 20 days) / 20]

Calculating the above expression:
- Time taken by A = (1/3) * (20 / (work done by A in 20 days))
- Time taken by A = (1/3) * (20 / [(2/3) - (work done by B in 20 days)])

Simplifying the expression:
- Time taken by A = (1/3) * (20 / [(2/3) - (20 * [(1/30) - (work done by A in 1 day)])])

Substituting the values:
- Time taken by A = (1/3) * (20 / [(2/3) - (20 * [(1/30) - (work done by A in 1 day)])])
- Time taken by A = (1/3) * (20 / [(2/3) - (20 * [(1/30) - A/30])])

Simplifying further:
- Time taken by A