The number divisible by $7$is

A) subtract first three digits from last three digits

      $666-666=0$

    ‘0’ is divisible by 7

666666 can divide by 7

B) A number is divisible by $7$subtract twice the unit’s digit from the remaining.

 $777-7=770$it is divisible by 7

C) $747-474=273$

$27-2\times 3=27-6=21,\text{ it is divisible by 7.}$