For an A.P. $a = - 18.9, d = 2.5, a_{n} = 3.6,$ find $n$ .

n = 12

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n = 10

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n = 8

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n = 6

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Solution

The correct option is B $n = 10$
Here, $a = - 18.9, d = 2.5$ and $a_{n} = 3.6$ .
Therefore, $a_{n} = a + (n - 1) d$
$- 18.9 + (n - 1) \times 2.5 = 3.6$
$\Rightarrow (n - 1) \times 2.5 = 22.5$
$\Rightarrow n - 1 = 9$
$∴ n = 10$

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