Recently trailhead has introduced a very handy playground org for the tasks. Really I have no issue with it even I don't really know my user name and password for that playground org.
However, for this trail: https://trailhead.salesforce.com/reports_dashboards/reports_dashboards_appexchange I need to install a managed package into the target org which requires user login. But I still can't find my credential for that org... How can I walk around this issue?