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A number for which sum of all its factor is equal to twice the number is called as Perfect Number.
A given number for which sum of all its factors is equal to it's twice, the number is known as A Perfect Number 

Read the following examples to understand more about Perfect Numbers 

Examples 1 = Is given number 6 a prime number ? 
Answer = Find the factors of given number 6 
Factors of 6 = 1, 2, 3, 6. 
Now, add all the above factors. 
Sum = 1 + 2 + 3 + 6 = 12 
Now, you can calculate that twice of given number 6 = 12 
Since the sum of all the factors of 6 is equal to its twice 
So, we can say that 6 is a perfect number.