Write down any three-digit number (for example, 425). Examine why they work ?
Make a six-digit number by repeating these digits in the same order (425425). Your new number is divisible by 7, 11, and 13.

Answer

Note that `7 xx 11 xx 13 = 1001`. Take any three digit number say, abc. Then `abc xx 1001` = abcabc. Therefore, the six digit number abcabc is divisible by 7, 11 and 13.