Forumla for totalling a sum that requires both addition and multiplication

Hi there,

Complete beginner/novice here. I need help creating a calculation that allows me to have a total price sum that calculates the price of an installation option (example: greenhouse installation as a checkbox) with the quantity number (textfield option).

Previously I had created a textfield that calculated the price of structure length option(s) selected (as checkboxes) with the built-in sum+ option in Properties > Calculate. And that did exactly what I needed.

After duplicating the same text field for the installation price total, I need to create a different formula that factors in the selected installation option (an additon) and also adds the quantity typed in (a muplication).

For example, if someone order shelving for their order, the average value is £10 per item. But if the customer ordered 5 of those, it would essentially be £50 all together. But I would need both of those fields (installation average value / quantity) to total in the Installation price field (among all the other added options). 

Assuming this will need to be either a simplied field notion, a custom calculation script or a 'run a Javascript' option, how do I go about this?

Kind regards

John