873 - Math - Problem Solving

Question: If A and B work together, they will complete a job in 7.5 days. However, if A works alone and completes half the job and then B takes over and completes the remaining half alone, they will be able to complete the job in 20 days. How long will B alone take to do the job if A is more efficient than B?

1. 20 days
2. 40 days
3. 30 days
4. 24 days