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

and 

Notice that 28 is the common denominator.
First of all, rewrite both the fractions having common denominator 28.
Multiply the numerator and denominator of 

by 4 to find an equivalent fraction. 

× 

= 

Multiply the numerator and denominator of 

by 7 to find an equivalent fraction. 

× 

= 

Now add fractions. Add numerators and keep denominators the same.

+ 

= 

+ 

= 

is written in simplest form. 
So, the required sum is 

. 
ds
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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.