G4.1 Let $a$ be a 3-digit number. If the 6-digit number formed by putting $a$ at the end of the number 504 is divisible by 7,9 , and 11 , find the value of $a$.
Note that 504 is divisible by 7 and 9 . We look for a 3-digit number which is a multiple of 63 and that $504000+a$ is divisible by 11.504504 satisfied the condition.