Numbers are arranged by their place values. Place values give a quantity of value to a certain digit in a number. Given a number, the rightmost number is the ones place, followed by the tens place, hundreds, thousands, ten thousands, hundred thousands, millions, ten millions, hundred millions, as so on and so forth. The place value of a certain digit is 10 times bigger than its adjacent digit to the right.
So for this problem, all you have to do is multiply the number of units of each place value, then multiple them all by 10.
(2(10) + 1(1))×10 21 × 10 = 210
Therefore, the number is equivalent and written as 210.
