How to make calculator on the purchase page?

Kind time of days, prompt how to make a calculator for calculations on the purchases page? I know that there is a lot of information, but I'm asking for help from you, those who did and knows what's what.
April 3rd 20 at 18:38
2 answers
April 3rd 20 at 18:40
If as you say you need to touch the code, thank Google and see how much poke you want.
Thank you - Gertrude commented on April 3rd 20 at 18:43
April 3rd 20 at 18:42

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