Excuse me, are you asking about the rule of missing numbers on envelopes in grade one? Looking for a rule in the envelope of first-year mathematics is actually looking for a rule to fill in numbers. There are four rules:

1, in descending order.

2. They are arranged in the order from big to small.

3, is to fill in the singular.

4. Fill in even numbers. Every problem has its own rules. Look at the questions carefully and do it when you find the rules.