tell whether each number is prime or composite?

Question

47
117
87

Answer 1

a number is prime if it has no factors other than 1 and itself.

you try various numbers to see if they divide evenly into your numbers

you can also do divisiblity tests. the test for 3 is very easy. Add up the digits in the number you are looking at. If they add up to a number divisible by 3, your original number is divisable by 3, and therefore, composite.

117=1+1+7=9 which is divisible by 3, so composite

87=8+7 =15 which is divisible by 3 (if you didn't know that, add !+5=6 you just keep doing it tell you get an easy number), so 87 is divisible by 3, so composite

47=4+7 =11, so not divisible by 3, or 2 because it is not even, or 5 because it doesn't end in 5 or 0, so you try dividing by 7, 11, 13 , 17, 19, 23, and find that none of them go into it. So it is prime.

Also by grade 7, you should have memorized the lists of primes to 100. Saves a lot of work to do so.

Answer 2

A number is prime if it cannot be divided by any number other than itself and a unit.
When working with positive integers, the only unit is 1.

For example, you can tell that 117 is divisible by 3 (which is not a unit and not 117); therefore, 117 is not prime.

Any number that ends with an even digit is divisible by 2 (e.g., 25,692 is divisible by, therefore it is not prime).
Add the value of each digit, keep repeating (add the value of each digit in the sum) until you get a number small enough; if that number is divisible by three, then the original number is divisible by 3.

e.g., 117: 1+1+7 = 9 which is divible by 3, therefore 117 is divisible by 3.

e.g., 774,956,253:
7+7+4+9+5+6+2+5+3= 48:
4+8=12:
1+2=3
Therefore 774,956,253 is divisible by 3.

If the number ends with a 0 or 5, then it is divible by 5.

And so on (there are tricks for higher primes, they get a little more complicated).

If a number is prime, the only sure way to prove it is to try to divide it by all the primes from 2 to the square root of the number.

e.g., 47 has a square root between 6 (36) and 7 (49).
The primes up to 6 are: 2, 3, 5,

We know that 47 is not divisible by 2 (47 is odd)
We know 47 is not divisible by 3 (4+7=11: 1+1=2 not divible by 3)
We know 47 is not divisible by 5 (does not end in 0 or 5)

Therefore 47 is prime.

Answer 3

87 & 117 are composite.
Here's a trick: if the sums of the digits of an integer is divisible by 3 or 9, then the number is divisible by 3 or 9 respectively.

8 + 7 = 15
87 is divisible by 3. (3 * 27)

1 + 1 + 7 = 9
117 is divisible by 9. (9 * 13)

There's even a method to find out if the number is divisible by 11 as well.

Answer 4

47 is prime, 117 and 87 both are divisible by 3

Answer 5

prime numbers have only two factors 1 and itself

Composite numbers has other factors beside 1 and itself.

47 = prime

87 composite

117 Composite

Click on the URL below for additional information concerning prime numbers

primes.utm.edu

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Answer 6

i don't agree with ANYTHING conservapedia says. those are the same folks who want to re-write the bible to eliminate the "liberal bias" in the king james translation. as far as partisanship goes.... well, if you're the REAL sophie [edit: now it's "lib nemesis"? don't you know that multiple accounts are a violation of the TOS?]... let's just say some of the things you've written here on Y!A have convinced this liberal that there is no going back, and no going forward without bloodshed. i'm through being reasonable and talking nice. total war. scorched earth. no retreat, no surrender, and no quarter given. conservatism must be crushed like the disease that it is. americans wiped out small pox, and nearly wiped out polio. conservatism will be the next great enemy of the people to be defeated once and for all.

Answer 7

47 prime
117 composite
87 composite

Answer 8

47 is prime. 87 is prime, and 117 is composite.

Answer 9

yes no no