How do I get this number to Multiply with another input?

How do I get this number to Multiply with another input?

Eg/

If I had one question asking how many items, then how do get it to multiply with another question which asks about price?

You can do that by adding Form calculation widget to your form.

Please refer to this helpful guide : 

~How to Perform Form Calculation Using a Widget.

~How to Add a Widget to Your Form

Please give it a try and let us know if you need any further assistance.