Write the following set in roster form also write its cardinal number
$A = {x / x = 3 n; n \in N and n < 8}$

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Solution

$x = 3 n$
where $n$ is a natural number and $n < 8$

So, $n$ can be any digit from $1, 2, 3, 4, 5, 6, 7$

So, in roster form
$A = {3, 6, 9, 12, 15, 18, 21}$

Cardinal number = no. of distinct elements in the Set $= 7$

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