What is the quantity of thousands in six thousand seven hundred twenty one? Six? Correct. How many hundreds? Seven? Wrong. If every thousand has ten hundreds, there are sixty seven hundreds here. And six hundred seventy two tens.
In other words, 6721 ones have 672 tens, 67 hundreds and 6 thousands. Without this knowledge firmly established, how could we subtract?
The questions like this teach to use counting words and appreciate the digits.
For further practicing, please find and print out the digits from any irrational number - it can be pi, e, square root of 2, etc. And, for everything sacred, don't call them decimals. 6721 is a decimal number too.