Trying to create a "product (x)" value in a text field. It won't let me select two individual fields to create the formula. I can only "select all" or "deselect all."

In my form I have four columns and multiple rows. For each row I want the first and third columns to multiply and the answer inserted in the fourth column. When clicking on the field in the fourth column, it will not let me click the individual fields in the first and third column to create the formula. I can only click on select all or deselect all.

Also, I want to do this in all of the rows. If there is a solution to my first question, is there then a fast way to do this in the remaining rows?