Hello E.M.

In a Qweb report, you can define a variable and use it for computation same as we can do it in python code.

<tbody class="invoice_tbody">
    <t t-set="total_discount" t-value="0"/>
    <tr t-foreach="o.invoice_line" t-as="l">
        <t t-set="total_discount" t-value="total_discount + (l.price_unit * l.quantity * l.discount)"/>
        <td><span t-field="l.name"/></td>
        <td>
            <span t-field="l.quantity"/>
            <span t-field="l.uos_id" groups="product.group_uom"/>
        </td>
        <td class="text-right">
            <span t-field="l.price_unit"/>
        </td>
        <td class="text-right" groups="sale.group_discount_per_so_line"><span t-field="l.discount"/></td>
        <td class="text-right">
            <span t-esc="', '.join(map(lambda x: x.description, l.invoice_line_tax_id))"/>
        </td>
         <td class="text-right">
            <span t-field="l.price_subtotal" t-field-options="{&quot;widget&quot;: &quot;monetary&quot;, &quot;display_currency&quot;: &quot;o.currency_id&quot;}"/>
        </td>
    </tr>
Total Discount: <span t-esc="total_discount"/>
</tbody>

As you can see in the code (highlighted code), using t-set you can define a variable and can assign or add the value based on calculation.

Documentation: Set a variable in Qweb reports