Five test scores have a mean of 7 marks. Four of the scores are 6, 7, 8 and 9 marks. What is the fifth score?

الشرح

Five scores with a mean of 7 must total 35 marks, and the four known scores add to 6 + 7 + 8 + 9 = 30, so the fifth score is 35 - 30 = 5 marks. Answering 7 marks copies the mean without checking the totals, yet the four known scores already average more than 7, so the missing score has to sit below the mean to pull it back down.