Scenario: I have a contract to install 100 widgets and I have a machine (resource) that installs 10 widgets per day. I want to be able to show 100 widgets that need to be installed, and when I assign the 10 widget per day resource, MS Project will automatically return a duration of 10 days. And if I change the quantity to 110 widgets, MSP will adjust the duration, or if I add a 2nd widget installer with a fixed production rate, the duration will adjust.
Is this possible? It seems like it should be a basic element...

In Microsoft Project, Material resources can either be consumed at a Fixed Rate or a Variable Rate. What you need is to assign the Widgets at a Variable Rate. Set the Duration of the task to 10 days and assign the Widget resource at a rate of 10/day. Microsoft Project will then calculate the consumption at 100 widgets. If the Duration is increased to 15 days, the consumption will be 150 widgets.

In Microsoft Project, the software DOES NOT calculate the Duration of the task based on the consumption rate of the Material resource. It works the other way around. Hope this helps.