Step 1: Understanding the Question:
This time and work problem provides the completion time for a joint effort and for one individual, requiring us to calculate the solo completion time for the second individual.
Step 2: Key Formula or Approach:
An individual finishing a job in \(x\) days has a daily work rate of \(\frac{1}{x}\).
The joint work rate is the sum of individual rates: \(\text{Rate}(A+B) = \text{Rate}(A) + \text{Rate}(B)\).
Step 3: Detailed Explanation:
Since A and B together finish the job in 12 days, their combined daily work rate is:
\[ \text{Rate}(A+B) = \frac{1}{12} \]
A working alone takes 20 days, meaning A's daily work rate is:
\[ \text{Rate}(A) = \frac{1}{20} \]
Let B's daily work rate be \(\frac{1}{x}\), with \(x\) representing B's solo completion time.
\[ \text{Rate}(B) = \text{Rate}(A+B) - \text{Rate}(A) \]
\[ \text{Rate}(B) = \frac{1}{12} - \frac{1}{20} \]
Finding the least common multiple (LCM) of 12 and 20, which is 60, allows us to subtract the fractions:
\[ \text{Rate}(B) = \frac{5}{60} - \frac{3}{60} = \frac{2}{60} = \frac{1}{30} \]
Because B completes \(\frac{1}{30}\) of the work per day, B will take 30 days to finish the job alone.
Step 4: Final Answer:
The correct choice is (B).