Answer:
Fifteen workers finish a project in 60 days. It means the project requires 15*60 = 900 man-days. If the project is to be completed in 36 days it will need 900 workers -days/36 days = 25 workers.
But make sure that there ample space to move about for 25 workers. If there is not much space available they can do the work in 2 shifts and complete the project in time.