Erdős conjecture on arithmetic progressions

Erdős conjecture on arithmetic progressions is an idea from a famous math guy named Paul Erdős. He said that if you have a really long list of numbers, there will always be a group of three numbers (called an arithmetic progression) that adds up to a multiple of 10. For example, 5, 10, 15 make a 3-number arithmetic progression that adds up to 30, which is a multiple of 10. So Erdős said that this will always be true no matter how big a list of numbers you have. Nobody has been able to completely prove his idea yet, but lots of smart people are still looking into it.