Subtract Fractions with Unlike Denominators
Use the least common denominator to find the difference between fractions with unlike denominators.
For example: Let's find the difference of:

and 

Notice that 6 is the common denominator.
First of all, rewrite the fraction having common denominator 6.
Multiply the numerator and denominator of 

by 3 to find an equivalent fraction. 

× 

= 

Now subtract fractions. Subtract numerators and keep denominators the same.

– 

= 

– 

= 

is not written in simplest form. 

÷ 

= 

So, the required difference is 

. 
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Help
The correct answer is
Remember :
The smallest number is the one that comes first while counting.
Solution :
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.