tell whether each number is prime or composite?

Question

93
121
7

Answer 1

composite
composite
prime

Answer 2

93 is composite because it is divisible by 3 and 31.
121 is composite because it is divisible by 11.
7 is prime because it is only divisible by 1 and 7, which is the definition of a prime number.

The following definition may help you in the future:

A prime number is any number that is divisible ONLY by the number 1 and itself. This is why 7 is a prime number. Other examples are 3, 5, 19, and 23. 2 is the only even prime number, every other prime number is odd.

A composite number is any number that isn't prime: this means it can be divided by 1, itself, and at least one other number. Some examples are 4 (divisible by 1, 4, and 2) and 27 (divisible by 1, 27, 3, and 9).

Some random facts and helpful hints:
- Any number ending is 5 is ALWAYS composite (except the number 5), because any number ending in 5 is divisible by 5.
- If a number is even, it is ALWAYS composite, except for two, which is the only even prime number.
- 1 is the only number that is neither prime nor composite.
- Prime numbers usually end in 1, 3, 7, or 9, but ALWAYS check by dividing by 3, 7, or 9 to make sure. That is the fastest way to find out if it is composite. If it can be divided evenly by any of those numbers, it is composite. Remember that if it is even or ends in 5, it is automatically composite.

I hope this helped! :)

Answer 3

93 is divisible by 3 since the sum of its digits is 12,
which is divisible by 3.
So 93 is composite.

121 = 11*11,
so 121 is composite.

7 is a prime. Just check all possible prime
factors less than 7. None divide 7.

Answer 4

93 and 121 are composite because 93 can be divided by 1,3 & 93
121 can be divided by 1, 11 & 121

7 is prime because it can only be divided by itself and 1.


You can check by the rules of divisibility:
2- it must be even

3- the sum of it's # must be divisible by 3
e.g. 3546. 3+5+4+6=18 and 18 is divisible by 3 so the # is divisible by 3.

4- the last 2 digits must be divisible by 4

5- it must end with a 5 or 0

6- it must be divisible by 2 AND 3

7- seperate the first # from the rest of the #. double the 1st # then subtract it from the rest of the number. the result must be divisible by 7 or else it's not divisible by 7.
e.g 29. 9x2=18. 2-18= -16 which is not divisible by 7.

8- the last 3# must be divisible by 8

9-the sum of it's digits must be divisible by 9. (same as 3 but u must use 9 instead)

10- it must end with a 0

11- you subtract and add the digits. Start with subtraction. the result must be divisible by 11 or else the # isn't divisible by 11
e.g. 4892847. 4-8+9-2+8-4+7=14 so it isn't divisible by 11

12- the # must be divisible by 3 AND 4.

Answer 5

you should know that 7 is prime. It has no other factors but itself and 1.

do the divisiblitly test for 3 on the other numbers. if their digits add up to 3, they are divisible by 3


9+3==12 12dividedby3=4, so 93 is divisible by 3 ---3x31=93

121=1+2+1=4, so not divisible by 3, 2 because not even, not 5 because doesn't end in 5 or 0. try 7 , 11, 13 ----- do it yourself, and you will find that one of them works

Also is helpful to memorize list of primes to 100 . should have that done in grade 7. also list of squares, and you would know 11x11 =121

Answer 6

You need to test each number by dividing them by primes.

You don't need to test divisibility by 2, 3, 4, 5, 6, etc. If a number is not divisible by 2, then it is not divisible by 4 or 6. Two is a factor of those composite numbers, and if a number is not divisible by a factor, then it is not divisible by that number.

So, you now only need to check the primes: 2, 3, 5, 7, 11, 13, 17, etc. With enough practice, you'll know the list of primes you need.

Look at 93:
2 does not divide 93.
3 divides 93 (resulting in 31).
The definition of a prime number is a number that has 1 and itself as factors. Since you know that 3 and 31 are factors, you know that 93 is not prime.

Look at 121:
2 does not divide 121.
3 does not divide 121.
5 does not divide 121.
7 does not divide 121.
11 divides into 121 (resulting in 11). So, you know that 121 is not prime.

Look at 7:
2 does not divide 7.
3 does not divide 7.
5 does not divide 7.
The only factors of 7 are 1 and 7, so 7 is prime.

Answer 7

Well, I'm going to be like everyone else and say 93 is composite, 121 is composite, and 7 is prime. All even numbers besides 2 are composite (just so you know for future reference).

Answer 8

93 is composite. It is divisible by 1, 3, 31, and itself

121 is also composite. It is divisible by 1, 11, and itself.

However, 7 is prime. It is only divisible by itself and 1.

Answer 9

7: Prime Number
93: Composite
121: Composite

Answer 10

composite
composite
prime

-An easy way to figure out whether numbers are divisible by 3 is to add the integers and see if that is divisible by 3. (1+2+1=3 or 9+3=12)
-To see if a number is divisible by 4, you see if the last two digits are divisible by 4..(54320 is divisible by 4, because 20 is divisible by 4)

Those help alot in problems like these..

Answer 11

93 is a composite , 121 is a prime and 7 is a prime