Playing With Numbers — MATHS STD 6 — Question

$4)84(21$
$\underline {8}$
$4$
$\underline{4}$
$0$
$84$ is divisible by $4$
So $21084$ is also divisible by $4$ because a no. is divisible by $4$ if the no. formed by its last two digits $($ i.e ones and tens $)$ is divisible by $4.$
$ii.$ Divisibility by $8$
The number formed by last three digits $= 084 = 84 (8)84(10)$
$\underline {8}$
$4$
$\underline{0}$
$4$
$\because$ Remainder is not $0.$
$\therefore$ $84$ is not divisible by $8.$
$\therefore$ $21084$ is not divisible by $8$ because a no. is divisible by $8$ only if the no. formed by its last three digits is divisible by $8.$