How can you compute grand totals in an aggregate view
You have to use Views Calc module.
The project page says:

This module adds simple calculations to a views table. Requires the
Views module.
You can SUM, AVERAGE, COUNT, MIN, MAX, or AVG selected columns. A plug
in theme makes the view look more spreadsheet-like (right justifies
numeric fields, shades calculated columns and rows, and underlines
calculated rows).

For more information go through the module documentation.