How to tell if a number is prime: estimate the square root. In this case, we know the square root of 31 is between 5 and 6 and it isn’t divisible by either of those. If it is divisible by a number higher than it’s square root, the other factor must be lower. Is it divisible by 4 or 2? no, because it isn’t an even number. It’s not divisible by 3, because there’s a remainder of 1 if you try. You’ve eliminated all numbers less than the square root, so it is prime
