General Clauses Act, 1897
12. Duty to be taken pro rata in enactments
Where, by any enactment now in force or hereafter to be in
force, any duty of customs or excise, or excise, or in the nature thereof, is leviable on any given quantity, by weight, measure or value
of any goods or merchandise, then a like duty is leviable
according to the same rate on any greater or less quantity.