1.13,the formula n(n-1)/2 should be used ,that is n(n-1)/2=78
Explaination:every player will play (n-1) matches,and there are n player in all,however,if we just use n(n-1),we count every matches twice and should be divided by 2.
2.7,
Explaination:the difference between a four-digit number and its inverted sequence number is required,therefor ,the difference is symmetric.For example,the difference between 1234 and its inverted sequence number 4321 is 3113