In a class quiz, 3 pupils scored 1 mark, 5 pupils scored 2 marks and 4 pupils scored 3 marks. What is the total number of marks scored by the whole group?

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Each score is earned as many times as its group is large, so the marks come to 3 x 1 + 5 x 2 + 4 x 3 = 3 + 10 + 12 = 25. Adding the six numbers in the description gives 18, but the group sizes count pupils rather than marks, so they must multiply the scores instead of joining them in the sum.