A Thinking Man (D5)

Professor Percent was a maths lecturer with an interest for new ways to express mathematical expressions. The traditional symbols (+, -, *, /, etc) were not enough anymore, to convey his superior numeric operations, so he had to invent new symbols, and only a superior brain would be able to understand the need for his new symbols.

The first symbol he invented was §; between two numbers, it meant that, if the first number was greater than the second, then the second should be subtracted from the first one; otherwise the two numbers should be added. Therefore 5 § 2 = 3, while 2 § 5 = 7.

The poor people that had to put up with this were, of course, his students. In the last test they were faced with:

5 ¿ 2 = 27
6 ¿ 3 = 27
8 ¿ 4 = 36

and also with:
5 ¤ 2 = 15
6 ¤ 4 = 12
3 ¤ 8 = 40

What are the meanings of the symbols ¿ and ¤?