There are some fractions whose denominator can be written as a power of 10. To convert such fractions to decimals:
1) Find an equivalent fraction with denominator 10, 100, or 1000 etc.
2) Then, counting from extreme right to left, place the decimal point in the numerator after as many digits as there are zeros in the denominator.
If the numerator has fewer digits, then add the desired number of zeros to the left of the numerator, and then place the decimal point.
We can multiply the denominator, 5, by 2 to get 10.

= 

= 

Observe that there is one zero in the denominator.
So, counting from right to left, place the decimal point in the numerator after one digit.
So, 

= 

=  0.8 
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The correct answer is
The smallest number is the one that comes first while counting.
To arrange the given numbers in order from smallest to greatest, find the smallest number among all the given numbers.
21,27,23
21 is the smallest number.