Here are two formulas from the book
Revisiting the formula that ignored size variance:
50 PBIs (total) / 8 PBIs per Sprint (velocity) 3 2-week Sprints 5 12.5 weeks
When you take into account the size variance of the Product Backlog items by applying points, you end up with a revised end date:
160 points (total) / 19 points per Sprint (velocity) 3 2-week Sprints 5 16.8 weeks
I understand the calculations should be as follows:-