Algorithm+proof+runtime

You are running the end of day processing for a financial institution. You have n different database tasks that have to be performed. Each task i consists of two parts. The first part is a database update that takes expected time wi minutes followed by post-processing that takes expected time ri minutes. Only one task may update the database at a time, and tasks may not be interrupted once started. The post-processing can all occur in parallel. You may not leave until all the tasks are complete. You want to know the order you should schedule that the n tasks to do their database updates so that you can leave as early as possible.