Angular: Scoring a checkbox or Radio Button. Clearing the answer if answer is changed

I am having problems with the scoring of my quiz.Currently i can add up the number of right and wrong answers upon submit of each question.However, when the person tries to go back and change his answer, the new score is added on top of the old one.That means i can have 2 Wrong answers for each question, which leads to a wrong answer. I am a beginner. Please help.

Quiz