How do I use sum function in this case????????????

Kurumi0418,

Please take a look at the following similar thread about how to use sum function in custom column. If you have questions about the code, please share sample data of your tables and post expected result based on sample data here.

https://community.powerbi.com/t5/Desktop/How-to-Sum-columns-in-edit-queries/td-p/75060

Regards,
Lydia