ChristopherPorter1 --
Really good question, my friend. Before you enter either Duration or Work for the task in question, double-click the task and then click the Advanced tab of the Task Information dialog. Click the Task Type pick list and select the Fixed Duration option,
then click the OK button. This action "locks" or fixes the Duration of the task so that it will not change when you adjust the Work value. For the task in question, enter a Duration value of 90 days and then press the Enter key on your computer keyboard.
Next, right-click anywhere in the Gantt Chart pane and select the Show Split item on the shortcut menu. In the Gantt Chart pane, select the task in question. In the Task Form pane, select the name of the resource you want to assign to the task and then
enter 60h of Work. Click the OK button in the Task Form pane to make the resource assignment.
The result is that you have a task with a Duration of 90 days and a resource assigned to perform approximately 60 hours of Work on the task. Hope this helps.