Q217 / 270 · NDA 2011 (Phase 2) · Mathematics

The mean of 7 observations is 10 and that of 3 observations is 5 . What is the mean of all the 10 observations?

  • A
    $15$
  • B
    $10$
  • Correct answer
    C
    $8.5$
  • D
    $7.5$

Solution

$\begin{aligned} \text { Total of } 7 \text { observations } & =7 \times 10=70 \\ \text { Total of } 3 \text { observations } & =3 \times 5=15 \\ \text { Total of } 10 \text { observations } & =70+15=85 \\ \therefore \quad \text { Mean of } 10 \text { observations } & =\frac{85}{10}=8.5 .\end{aligned}$

Hence, the correct answer is option: 'C'.

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