title_body: How the idea of 30 ml, 60 ml and 90 ml drinks get started?
upvoted_answer: The British used the Imperial system of measures from 1824 and India was part of the British Empire until 1947. 1 British Imperial fluid ounce is equal to 28.41 ml, which is close to 30 ml. Maybe this practice evolved from the specification that 1 peg equaled 1 imp. oz. and was rounded up to 30-ml after metrification. 
  
  Standard Indian-made liquor is packaged in 90-, 120-, 180-, 360-, and 750-ml bottles to make it easier to divide equally into pegs.