Write $ 8.21 \times {10^{ - 3}} $ as an ordinary number.

Hint: In mathematics, there are several different types of numbers. These include whole numbers, rational and irrational numbers, and real and imaginary numbers.An ordinary number is the expanded version of the standard form number.

Complete step-by-step answer:
The difference between the ordinary number and ordinal number. Ordinal number is the number which tells the position of the object or something in the list. For Example first, second, third, etc. It simply tells us the rank or the position of something in the group.
Step 1 Observe the given number and power of $ 10 $
Here given that the power to \[10\] is $ ( - 3) $
Step 2 Write as the division of $ 10 $ three times instead of multiplication.
\[8.21 \times {10^{ - 3}} = 8.21 \div 10 \div 10 \div 10\]
Step 3 Do the multiplication by $ 10 $ one at a time
\[
  8.21 \times {10^{ - 3}} = 0.821 \div 10 \div 10 \\
  8.21 \times {10^{ - 3}} = 0.0821 \div 10 \\
  8.21 \times {10^{ - 3}} = 0.00821 \\
 \]

Note: Step 1: Think of the given power as being the positive.
Step 2 Write as many zeros, including one before the decimal and then write down the given number.In case of having positive power to ten, simply write zeros after the number. If number two is given to the power ten, then write two zeros after the number.
Also, remember the difference between the ordinary number and ordinal number. Ordinal number is the number which tells the position of the object or something in the list. For Example first, second, third, etc. It simply tells us the rank or the position of something in the group.Whereas, the ordinary numbers are the numbers which includes whole numbers, rational, irrational numbers and real and imaginary numbers.