【How-to】How much does 12 cup of salt weigh

Use of ‘a / an’:

(c) a/an is used in certain expressions of quantity with certain numbers and in expressions of price: e.g.: a lot of, a couple, Rs. 10 a kilo, a great many, a dozen, 50paise a dozen, a great deal of, a hundred, four times a day, a thousand, 60 km an hour, Before half, when half follows a whole number.