About on text change event and dropdown on change event

hello everyone,

On my page i have 3 fields . I am working on MVC:

1) Category Dropdown (Categories such as Milk, Tea Powder, Paper Cup..etc)

2) Quantity Textbox

3) PriceTextbox

For Eg : When Milk category is selected, I will enter 50L as quantity and it should auto bind 1000 in Price textbox

it should be only when MILK category is selected in dropdown.

Thank you in advance