There are two main approaches to adding numbers based on dropdown list selections in Excel:

1. Using SUMIF or SUMIFS:

This method works well when you have a fixed list of options in your dropdown and corresponding values to add. Here's how:

* Create your dropdown list: Define the list of options you want users to choose from.
* Create a value column (optional): In a separate column, list the numeric values that correspond to each dropdown option. This step is optional if the values are directly linked to the dropdown options themselves.
* Select the cell for the sum: Choose the cell where you want the total to be displayed.
* Enter the SUMIF or SUMIFS formula:
- SUMIF: If you're adding based on a single dropdown selection:
`=SUMIF(dropdown_cell_range, dropdown_option, value_cell_range)`
- Replace `dropdown_cell_range` with the cell reference or range containing the dropdown list.
- Replace `dropdown_option` with the specific option you want to sum against (text in quotes).
- Replace `value_cell_range` with the cell reference or range containing the corresponding values (optional, only if separate from dropdown options).
- SUMIFS: If you need to add based on selections from multiple dropdowns:
`=SUMIFS(value_cell_range, dropdown_cell_range1, dropdown_option1, dropdown_cell_range2, dropdown_option2, ...)`
- This formula allows you to check for matching criteria across multiple dropdown lists simultaneously.

2. Using XLOOKUP (Excel 365 only):

This method offers more flexibility and can handle dynamic data sources. Here's a basic example:

* Create your dropdown list: Define the list of options you want users to choose from.
* Create a data table (optional): If your values aren't directly tied to the dropdown options, create a table with two columns: one for the options (matching your dropdown list) and another for the corresponding values.
* Select the cell for the sum: Choose the cell where you want the total to be displayed.
* Enter the XLOOKUP formula:
`=SUM(XLOOKUP(dropdown_option, data_table_option_column, data_table_value_column))`
- Replace `dropdown_option` with a reference to the cell containing the user's selection from the dropdown list.
- Replace `data_table_option_column` with the cell reference or range containing the options in your data table (if used).
- Replace `data_table_value_column` with the cell reference or range containing the corresponding values in your data table (if used).

Remember:

- Adjust the cell references in the formulas to match your specific spreadsheet layout.
- Ensure your dropdown list options match the criteria used in the formulas.

These are two common approaches for adding numbers based on dropdown selections. The best method depends on your specific needs and Excel version.