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.