Which term of an A.P : 21, 42, 63,.... is 210 ?

9th

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10th

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12th

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11th

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Solution

The correct option is B 10th
Let $a = 21$
and $d = 42 - 21 = 21$
$∴ a_{n} = a + (n - 1) d$
$\Rightarrow 210 = 21 + (n - 1) 21$
$\Rightarrow 189 = (n - 1) 21$
$\Rightarrow 9 = (n - 1)$
$∴ n = 10$

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