list all the prime numbers between 45 & 80.?

Answer 1

First, eliminate the even numbers. They are not prime, because they are multiples of 2. This leaves us with:

45, 47, 49, 51, 53, 55, 57, 59, 61, 63, 65, 67, 69, 71, 73, 75, 77, 79.

Now, delete the multiples of 3, because they are not prime either. We delete 45, 51, 57, 63, 69, 75 leaving us with:

47, 49, 53, 55, 59, 61, 65, 67, 71, 73, 77, 79.

Finally, we delete the multiples of 5 that we can see, namely 55 and 65, and the multiples of 7, namely 49 and 77. The numbers left are the prime numbers from 45 to 80:

47, 53, 59, 61, 67, 71, 73, 79.

We can stop there, because if there was any multiple of 11 which we have not yet deleted, it would have to be at least 11 * 11 = 121 which is more than 80.

Answer 2

A prime number is a number that has only two factors, 1 and itself. 1 is not a prime number, because the two factors must be unique.

Start by eliminating all numbers within the range that are divisible by 2 (i.e., all the even numbers). Then eliminate the ones that are divisible by 3 (just count by 3's). The ones that are divisible by 4 will already be gone, since they are even. Continue in this fashion until you have eliminated all numbers that have any factors. The ones that are left are:

47, 53, 59, 61, 67, 71, 73, 79

Answer 3

47, 53, 59, 61, 67, 71, 73, 79

Neither 51 nor 57 are prime.
51 = 17*3
57 = 19*3

Answer 4

One way to check if a number is prime is to determine if it is divisible by 3 and the divisor is also a whole number.

The way to do this is to add the digits and see if the sum is a factor of 3.

Example: 51, 5+1=6, 6 is divisible by 3, therefore 51 is not a prime number.

Example: 111, 1+1+1=3, 3 is divisible by 3, therefore it is a composite number.

This may be a trick no one has ever taught you.

Answer 5

47, 53, 59, 61, 67, 71, 73, 79

Answer 6

47, 51, 53, 57, 59, 61, 67, 71, 73, 79

Answer 7

Is 80 A Prime Number