The three angles of the triangle correspond to 100, 300 and 500 respectively.

Then a number in the middle of the edge formed by "100 and 300" is 1000- 100-300=600.

A number in the middle of the edge formed by "100 and 500" is 1000- 100-500=400.

In fact, this problem is a mathematical problem of finding laws, and it is also an addition problem, as long as all three lines are 1200.

Find a routine method:

Numbering: To find a regular topic, we usually give a series of quantities in a certain order, and ask us to find a general rule according to these known quantities. Find out the rule, usually the serial number of the package. Therefore, it is easier to find the mystery by comparing variables with serial numbers.