hi everyone, i am new to the forum.
I have a question (maybe obvious to you!)
if for a task I have 100 hours available but for this project I can dedicate only 20% of the resource how do I get the correct task duration?

The overall Duration of the project would be around 62.5 days, depending on your company holidays. What you need to do is to create your task list using Auto Scheduled tasks, leave the Duration of each task at the default Duration of 1 day, and then set task dependencies on every task in the project. Next, apply the Resource Sheet view and enter the names of your project team members. These steps are absolutely MANDATORY.

After doing this, apply the Gantt Chart view and then right-click anywhere in the white part of the Gantt Chart screen and select the Show Split item. This will apply the Task Entry view with the Gantt Chart view in the top pane and the Task Form view in the bottom pane. In the Gantt Chart pane, select the first task. In the Task Form pane, select the name of each resource you want to assign to the task, set their Units value to 20%, and then enter how many hours of Work each resource will perform on the task. When finished, click the OK button in the Task Form pane and Microsoft Project will calculate the Duration of that task for you. Repeat this process for every task in the project.

Hope this helps.