SUM of Hours column

Hello, I have four columns:

Task | Hours | Assigned to | Total hours

The 'hours' column is how much time (hours) it will take to do the task, the 'assigned to' you can assign the task for more than one person.

Now I need the 'total hours' column to show me how much time it will take for a task (example: Task1 takes 1 hour but it is assigned to three people, so it need to be counted as 3 hours, 1 hours for each person). And in the end, a total sum of all the time of all tasks.

Can someone help me, please?

Thank you!