Respected sir

I would like to know. How do they calculate the GST tax for and in grocery market's i.e in vegetables shop.?  How to calculate the gst for a particular veg..   

For example..   consider tomatoes.   If a shopkeeper has 20 kg of tomatoes then how we calculate the GST amount for him for tomatoes ..      how to calculate for other vegetables.. please help me in clarifying this doubt...  you can also send the same in my gmail also.. mageshrajj22@gmail.com. 

Thankyou..