Even addition and subtraction are even numbers; Odd numbers and even numbers are added and subtracted to form odd numbers; Even the odd phase is even, and the odd phase is odd.

Whether you answered correctly, mistyped or didn't answer each question, the scores of 5,-1 and 1 are all odd. Then, after a person answers 30 questions, his score is 30 odd numbers and he must get an even number. Plus the basic score 15, he must get an odd number, which means everyone's score will be odd.