Q259 / 270 · NDA 2022 (Phase 1) · Mathematics

The marks obtained by 51 students in a class are in AP with its first term 4 and common difference 3 . What is the mean of the marks?

A
67
B
71
C
75
Correct answer
D
79

Solution

$\begin{aligned} & S_n=\frac{n}{2}[2 a+(n-1) d] \\ & =\frac{51}{2}[8+50 \times 3]=\frac{51 \times 158}{2}=51 \times 79\end{aligned}$ Mean $==\frac{\mathrm{S}_{\mathrm{n}}}{\mathrm{n}}=\frac{51 \times 79}{51}=79$

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

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