This problem is worked out by using the six figures given above and the figures in the car below.

Namely: the first car12-6 = 6; 15-6=9; 14-6=8; 19-6= 13; 16-6= 10; 1 1-6=5

So put 6 in the box of the first car; 9; 8; 13; 10; five

Fill in 4 for the second car; 7; 6; 1 1; 8; three

The subtraction of natural numbers is not closed.

It is closed unless the minuend is greater than the minuend. For example, 1 1 cannot subtract 26. In this case, use one of two methods:

(1) says that 26 cannot be subtracted from 1 1;

(2) If the answer is an integer, it means a negative number, so the result of 1 1 minus 26 is-15.

The subtraction of real numbers is defined as the addition of signed numbers. Specifically, one number subtracts the negative number of another number. And then we have three? π= 3 +(? π)。 This helps to avoid introducing "new" operators such as subtraction, thus maintaining the "simplicity" of real numbers.