How can you tell if a number is prime or compsite

A prime number doesn't have any factors besides one and itself. For example, 17 is a prime number because you can't multiply anything to get 17 besides 17 and 1 (excluding decimals).

Composite numbers are numbers that have other factors other than 1 and themselves. Example: 12 is a composite number because you can multiply 3 by 4 to get 12, or 2 by 6 to get 12, not just 12 times 1.

6 adults and 9 children went.

a + c = 15

12a + 9c = 153

12(6) + 9(9) = 153

72 + 81 = 153