Congruence theorem: the core formula: congruence congruence, sum congruence, difference congruence, and common multiples have periods. Remainder: "When a number is divided by 4 remainder 1, divided by 5 remainder 1 and then divided by 6 remainder 1", it is 1, that is, 60+ 1. And the same: "A number divided by 4 is greater than 3, divided by 5 is greater than 2, and divided by 6 is greater than 1", then taking 7 is 60+7. Difference: "A number divided by 4 is 1, divided by 5 is 2, and divided by 6 is 3", so take -3 and it is 60-3.

The so-called congruence, as the name implies, is that many numbers are divided by a number d, and the remainder is the same. The mathematical name is module. If a=6, b= 1 and d=5, then we say that a and b are modular d congruences. Because they all have the same remainder 1.