Question

classify the following numbers into prime numbers and composite number.
28,53,35,83,14,41,23,32,87,19,91.​

Answer 1

$\Large\textrm{Formulae.}$

• $\textrm{Multiple checking.}$

Let's find out how to isolate prime and composite.

-Multiples of 2

All even numbers end in 0, 2, 4, 6, or 8.

-Multiples of 3

The digital root stands for the sum of all digits. The digital root is a multiple of 3 then the number is a multiple of 3.

-Multiples of 5

Multiples of 5 end with 0 or 5.

-Multiples of 7

The difference of the remaining digit and the first digit doubled leaves a multiple of 7.

$\textrm{ \bigstar About prime factorization.}$

A composite number has at least two prime factors. So, the least multiples can't exceed the square root. Here, we need to find the prime factors before 10.

1 is neither prime nor composite.

$\Large\textrm{Explained.}$

28 is an even number. (Composite)

53 is a prime.

35 is a multiple of 5 and 7. (Composite)

83 is a prime.

14 is a multiple of 2 and 7. (Composite)

41 is a prime.

23 is a prime.

32 is an even number. (Composite)

87 has a digital root of 15. (Composite)

19 is a prime.

91 is a multiple of 7. (Composite)

Prime: 19, 23, 41, 53, 83.

Composite: 14, 28, 32, 35, 87, 91.