834 - Math - Problem Solving

Question: Two workers A and B manufactured a batch of identical parts. A worked for 2 hours and B worked for 5 hours and they did half the job. Then they worked together for another 3 hours and they had to do (1/20)th of the job. How much time does B take to complete the job, if he worked alone?

1. 24 hours
2. 12 hours
3. 15 hours
4. 30 hours