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In a branch, 30 clerks average 60 marks and 20 officers average 70. The average of all 50 staff is:
A62
B65
C66
D64
Answer & Solution
Correct answer: D. 64
1. A plain average of 60 and 70 would be wrong because the two groups differ in size.
2. Work with totals instead: the clerks total 30 x 60 = 1800.
3. The officers total 20 x 70 = 1400.
4. Together that is 3200 marks over 50 people.
5. So the combined average is 3200 / 50 = 64. Answering 65 is exactly the unweighted-average trap.
_Source: NCERT Class 10 Maths, Ch 13 'Statistics', S13.2 Mean of Grouped Data_
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