Is 842 a Prime Number?

There are two types of numbers, mostly —

Prime numbers and composite numbers, depending on the number of factors.

A prime number is a natural number that is divisible only by 1 and itself.

For example, 3 is a prime number because it is divisible by 1 and itself.

A composite number is a positive number that is divisible by more than two numbers.

For example, 6 is divisible by 1, 2, 3, and 6, making it a composite number.

Prime numbers follow a few properties like: 

• Prime numbers are positive numbers always greater than 1. 
   
• 2 is the only even prime number. 
   
• They have only two factors: 1 and the number itself.
   
• Any two distinct prime numbers are co-prime numbers because they have only one common factor, which is 1.

As 842 has more than two factors, it is not a prime number.

The characteristic of a prime number is that it has only two divisors: 1 and itself. Since 842 has more than two factors, it is not a prime number. A few methods are used to distinguish between prime and composite numbers. Some methods include:

• Counting Divisors Method 
   
• Divisibility Test 
   
• Prime Number Chart 
   
• Prime Factorization

The method in which we count the number of divisors to categorize the numbers as prime or composite is called the counting divisors method. Based on the count of the divisors, we categorize prime and composite numbers.

• If there is a total count of only 2 divisors, then the number would be prime.

• If the count is more than 2, then the number is composite.

Let’s check whether 842 is prime or composite.

Step 1: All numbers are divisible by 1 and themselves.

Step 2: Divide 842 by 2. It is divisible by 2, so 2 is a factor of 842.

Step 3: Divide 842 by 3. It is not divisible by 3, so 3 is not a factor of 842.

Step 4: You can simplify checking divisors up to 842 by finding the root value. We then need to only check divisors up to the root value.

Step 5: When we divide 842 by 2, 421, and other factors, it is divisible by 2 and 421.

Since 842 has more than 2 divisors, it is a composite number.

We use a set of rules to check whether a number is divisible by another number completely or not. It is called the Divisibility Test Method.

Divisibility by 2: The number in the ones' place value is 2, which is an even number, meaning that 842 is divisible by 2. 

Divisibility by 3: The sum of the digits in the number 842 is 14. Since 14 is not divisible by 3, 842 is also not divisible by 3.

Divisibility by 5: The unit’s place digit is 2. Therefore, 842 is not divisible by 5. 

Divisibility by 7: The last digit in 842 is 2. To check divisibility by 7, double the last digit (2 × 2 = 4). Then, subtract it from the rest of the number (84 - 4 = 80). Since 80 is not divisible by 7, 842 is also not divisible by 7. 

Divisibility by 11: The sum of the digits in odd positions is 10, and the sum of the digits in even positions is 4. The difference is 6, so 842 is not divisible by 11.

Since 842 is divisible only by 2, 421, and itself, it has more than two factors, making it a composite number.

The prime number chart is a tool created by using a method called “The Sieve of Eratosthenes.” In this method, we follow the following steps.

Step 1: Write 1 to 1000 in rows and columns.

Step 2: Leave 1 without coloring or crossing, as it is neither prime nor composite.

Step 3: Mark 2 because it is a prime number and cross out all the multiples of 2.

Step 4: Mark 3 because it is a prime number and cross out all the multiples of 3.

Step 5: Repeat this process until you reach the table consisting of marked and crossed boxes, except 1. Through this process, we will have a list of prime numbers.

842 is not present in the list of prime numbers, so it is a composite number.

Prime factorization is a process of breaking down a number into prime factors. Then multiply those factors to obtain the original number.

Step 1: We can write 842 as 2 × 421.

Step 2: In 2 × 421, 421 is a prime number. Now we get the product consisting of prime numbers.

Hence, the prime factorization of 842 is 2 × 421.

• Composite numbers: Natural numbers greater than 1 that are divisible by more than 2 numbers are called composite numbers. For example, 12 is a composite number because 12 is divisible by 1, 2, 3, 4, 6, and 12.

• Twin primes: The pair of prime numbers whose difference is 2 are called twin prime numbers. For example, (11, 13) is a pair of twin primes because the difference between 13 and 11 is 2.

• Factors: The numbers that divide the number exactly without leaving a remainder are called factors. For example, the factors of 4 are 1, 2, and 4 because they divide 4 completely.

• Prime number: A number greater than 1 that has no positive divisors other than 1 and itself.

• Divisibility rules: A set of rules that help determine whether one number is divisible by another without performing division.