3, 9, 27, 81, 243
For this kind of question, I would suggest looking up a table of prime numbers. As an alternative, you can try to find factors for each of the numbers - if it has a factor, it is NOT a prime. For this range of numbers, testing for prime numbers up to 13 is appropriate. (If 17 is a factor of one of these numbers, the other factor is less than 17, so you would already have found it before you reach 17.)
2 and 7 are prime numbers less than 10 that are factors of 42 and 70.
3,9,27,81,243,729
1
When they have a common prime factor. When their GCF is greater than 1.
For this kind of question, I would suggest looking up a table of prime numbers. As an alternative, you can try to find factors for each of the numbers - if it has a factor, it is NOT a prime. For this range of numbers, testing for prime numbers up to 13 is appropriate. (If 17 is a factor of one of these numbers, the other factor is less than 17, so you would already have found it before you reach 17.)
2 and 7 are prime numbers less than 10 that are factors of 42 and 70.
210
3,9,27,81,243,729
3, 9, 27, 81, 243, 729
Use a factor tree. 1955 391,5 23,17,5
1
When they have a common prime factor. When their GCF is greater than 1.
31
To find prime numbers less than 100, the sieve of eratosthenes filters out 1 and all multiples of 2, 3, 5, and 7. All remaining numbers less than 100 are primes.
3, 9, 27, 81, 243
2,3,5,7.2*3*5*7=210