Students from 3 classes attend a mathematics exam.
The average marks in class I is 83.
The average marks in class II is 76
The average marks in class III is 85.
The average marks of all students in classes I and II is 79. The average marks of all students in classes II and III is 81. What is the average for all the three classes?

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81.5

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81.33

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Solution

The correct option is B 81.5

So, the ratio of students is 3:4:5
Required average $= \frac{(3 \times 83) + (4 \times 76) + (5 \times 85)}{12} = 81.5$

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The correct option is B 81.5 So, the ratio of students is 3:4:5 Required average =(3×83)+(4×76)+(5×85)12=81.5

The correct option is B 81.5

So, the ratio of students is 3:4:5
Required average $= \frac{(3 \times 83) + (4 \times 76) + (5 \times 85)}{12} = 81.5$