Hi @exaud1993,
You could design a survey form with a matrix question type where you calculate the last column as shown in the image below:
In the survey tab of your xlsform:
In the choices tab of your xlsform:
In the setting tab of your xlsform:
Data entry screen as seen in Enketo:
Reference xlsform:
Matrix Questions (Calculation).xls (44.5 KB)
Have a great day!