if you took 30ml out of a 60 ml milk thing and added it to the tea you would have 2/3 tea 1/3 milk so when you take 30ml from the tea you would take 20ml tea and 10ml milk meaning you would have 20ml milk remaining and 20ml tea and 10ml of the original milk in to the milk so basically you only add 20ml to each.
if you took 30ml out of a 60 ml milk thing and added it to the tea you would have 2/3 tea 1/3 milk so when you take 30ml from the tea you would take 20ml tea and 10ml milk meaning you would have 20ml milk remaining and 20ml tea and 10ml of the original milk in to the milk so basically you only add 20ml to each.