Help! I'm stuck with this formula

I want to make an app so you can quickly see how much you need to produce something else through recycling. But I cannot seem to get the formula right.

I want to make something like this:

Textbox.1 (number of recyclable objects) Listpicker 1 (objects itself like old cars, cans etc.)

Textbox.2 (number of produced objects)   Listpicker 2 (new products cars, bins, cans etc.)

Calculate button

so you can calculate al sorts of things to get kids interested in recycling.

the formula is supposed to be something like this:

textbox 2 = textbox 1 x 20.000 (listpicker 1 + and listpicker 2 are choosing this value (1 car makes 20.000 cans for example) how do i make this?)

like a sort of converter i tried to make something like this but i can't make it work can anyone tell me what i should do? because i dont know what i am doing wrong. I watched the tutorials and i have a know what i am doing but it is not working for me.

It would really help if you provided a screenshot of your relevant blocks, so we can see what you are trying to do, and where the problem may be.

simple example i am trying to get it when i have 1000mm and i want to know how many meters it is how can i tell my computer it has to convert it to :1000 when mm to m is true?