think of any 3 digit number ie 489

add the same number again to the end of it to make a 6 digit number ie 489489

now divide the number by 7,11 or 13 ie 489489 / 13 = 37653

now divide the number you have by any of the the other two remaining ones (7 or 11 in this case) ie 37653 / 7 = 5379

now divide that number by the last remaining number (11 in this case) ie 5379 / 11 = 489 !!!


apparently you always get back to the number you started with by doing this

maths eh - one of life's mysteries....