The primes—numbers that are only divisible by themselves and 1—are the most ... indirect approach. Around 300 BCE, Euclid proved that there are infinitely many prime numbers.

According to one account, he was once asked to name a prime number during a conversation. These numbers, which are only divisible by 1 and themselves, form the atoms of number theory, so to speak ...

Remember, a prime numbers can only be divided by itself and 1. You can use your knowledge of multiples (times tables) to eliminate numbers. Start with the smallest numbers and work your way up.