Discussion
A class of 25 students participated in a science test. 10 students had an average (arithmetic mean) score of 80. The other students had an average score of 60. What is the average score of the whole class?
Answer: A
The average is 68.
Ten students average 80, giving a total of 800. The other 15 students average 60, giving 900. The total for 25 students is 1700.
\[ \frac{800+900}{25}=68 \]
78, 65 and 64 do not come out of this calculation.
This is a weighted average. Ten students bring 800 marks and the other fifteen bring 900, so the total is 1700 for 25 students. Since 80 and 60 are the two group averages, the class average must lie between them and nearer 60 because there are more students in that group, which gives 68.
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