Look for a pattern. In normal arithmetic, 5+3=8, which is the last digit of the number given by this "new addition". 9+1=10, which are the last two digits of the "new addition". 8+6=14, which again are the last two digits of the number they give. Finally, 5+4=9. So, each number they yield ends with the normal arithmetic. So, now you just need a pattern to find the first digit. 5-3 = 2, 9-1 = 8, 8-6 = 2, and 5-4 = 1. Hence, 7+3 could give 4 as the first digit (as 7-3 = 4 in normal addition). Then, it can be followed by 10, as 7+3=10. I would think 7+3 = 410. But, there are an infinite number of patterns. This is just one such pattern. I am sure there is a valid reason for 7+3 = 710.