Change number based on input of related button

I would like to implement an 'add water' function in my app. When the user clicks on the 'glass' button, the number '0' should change into 250 ml. And when the user wants to add a bottle of water, the number 250 should change into 750 ml. How do I set-up this functions in proto.io? 

I added a screenshot to clarify what I mean. 

Thank you in advance! 


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