Alice can complete a digital artwork project in 15 days, while Bob can finish the same project in 30 days. They work together for 3 days before Alice leaves the project. In how many more days will Bob finish the remaining work alone?

The correct answer is 21 days.

Step 1: Determine individual work rates per day.
Let the total project work be equal to 1 unit.
Alice can complete the project in 15 days, so Alice's 1-day work rate is:
$\frac{1}{15}$
Bob can complete the project in 30 days, so Bob's 1-day work rate is:
$\frac{1}{30}$

Step 2: Find their combined 1-day work rate.
When Alice and Bob work together, the portion of work they complete in 1 day is:
$\frac{1}{15}+\frac{1}{30}=\frac{2+1}{30}=\frac{3}{30}=\frac{1}{10}$

Step 3: Calculate the work completed in 3 days.
Alice and Bob work together for 3 days, so the fraction of work completed is:
$3\times \frac{1}{10}=\frac{3}{10}$

Step 4: Calculate the remaining work.
The fraction of the work left to be finished is:
$1-\frac{3}{10}=\frac{7}{10}$

Step 5: Calculate the time taken by Bob to finish the remaining work alone.
Bob works at a rate of $\frac{1}{30}$ of the project per day. The number of days Bob requires to finish $\frac{7}{10}$ of the work is:
$\text{Time}=\frac{\text{Remaining Work}}{\text{Bob's Work Rate}}=\frac{\frac{7}{10}}{\frac{1}{30}}=\frac{7}{10}\times 30=21\text{days}$

Therefore, Bob will finish the remaining work alone in 21 days.