AlgebraVery hardNumber of girls from class averages
Algebra practice question
In a class, the 20 boys have an average test mark of 60 and the girls have an average mark of 70. The average mark of the whole class is 66. How many girls are in the class?
Answer: 30
20(60) + 70g = 66(20 + g) gives 1200 + 70g = 1320 + 66g, so 4g = 120 and g = 30.
Common wrong answers
- If you answered 6
- 6 stops at 4g = 24.
- If you answered 20
- 20 assumes equal numbers, which would give an average of 65.
- If you answered 26
- 26 comes from 66 − 40, a misreading.
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