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I need help to make a machine that can add and remove number with the click of an button

Hi!
I’m doing a project in school about clean water and I need to make a website were you can learn how to save water fast. But I need some help so.
I would like make a site where people would be able to click a button next to a piece of text. The text would say fx “If you have meat free day once a week, then you can save 15.000 L of water”, and then if clicked the button next to the text, then it would add 15.000 L of saved water to the amount of water that you saved this week.
I am not able to do this. I tried doing it with Ecommerce, but I could not get the product page to show in the navigation menu.

Best regards
John from Denmark.