Formula dropdown

Hello, I have 5 fields: fieldname1, fieldname2, fieldname12, fieldname5,and fieldname13 (is the total).

The first <%fieldname1%> has 3 choise dropdown, with value 100, 150, 200
The second <%fieldname2%> has 4 choise dropdown, with value 5, 10, 15, 20
The thirt <%fieldname12%> has 4 choise dropdown, with value 100,200,300,400
The four <%fieldname5%> has 4 choise dropdown, with value 50,250,500,1000

And the <%fieldname13%> is the total

How to calculate <%fieldname12%> * <%fieldname5%> with the diferent choise dropdown with diferent value

Thank you very much

https://www.ads-software.com/plugins/calculated-fields-form/