Quadrilateral, triangle and circle represent a number respectively, and they contain each other to represent the addition or subtraction of two numbers. There is also a hidden rule that there is no negative number in primary schools, so it can only be greatly reduced.

① As can be seen from the first and second pictures, the sum of quadrilateral and circle is 3 and the difference is 1, so one quadrilateral and circle is 1 and the other is 2;

② As can be seen from the third and fourth pictures, the sum of quadrilateral and triangle is 6 and the difference is 4, so one of quadrilateral and triangle is 1 and the other is 5;

Synthesizing ① and ②, we can get quadrilateral representation 1, circle representation 2 and triangle representation 5; A large number set of decimals represents subtraction, and a large number set of decimals represents addition.

The last picture is a triangle circle, so the answer is 5-2=3.