What is 21000 rounded to the nearest ten thousand

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. 

The answer is 20000 because you make the 1 a zero