A number rounded to the nearest 10000, means that the last four digits of that number (on the left of the decimal point) are 0.
However, if the fourth-last digit to the left of the decimal point is 5 or greater, the number to the left of that is rounded to 1 number higher. For example, if you were rounding 145293 to the nearest 10000, it would be 150000.
However, the fourth-last digit to the left of the decimal point in this case is 1.
Therefore, 21000 rounded to the nearest 10000 is 20000.