How do you reduce 21?

To find sum of all the alternate digit in a given 7 digit number? How do you solve 2n plus 466? How to find Median of probability distribution? How many inches is in one rotation? What is the product of 585 and 13? How ml equals 1 L? Who were the talent tenth? What is the product of 7 and the qoutient of 81 and 9? Why must a driver sit 10-12 inches away from the steering wheel? What is it called when you replace a word with a symbol? What is 30 percent of 42000.00? What will the pressure be at the base of a 1 meter high column of water? What is the equivalent fractions of? What in inferential reading? What is 230 m converted to cm? Which large sample of individuals us not included in the civilian labor force? How many different selections of 4 books can be made from a bookcase displaying 12 books? What color is a hare? Is there such a thing as a gillion? What is 210 rounded to the nearest integer?