6. (10 points) Person A and Person B work together on a project, which can be completed in several days. If Person A completes half of the project alone, it will take 10 days less than if Person A and Person B work together to complete the entire project; if Person B completes half of the project alone, it will take 15 days more than if Person A and Person B work together to complete the entire project. How many days will it take for Person A and Person B to complete the entire project together? $\qquad$ days.