Browse · MATH
Printjmc
number theory senior
Problem
A four-digit integer and the four-digit integer obtained by reversing the order of the digits of are both divisible by 45. If is divisible by 7, what is the greatest possible value of ?
Solution
Let the integer obtained by reversing the digits of be . and are both divisible by , which means that they are both divisible by . Thus, they both have units digits of or . If one has a units digit of , the other will have a leading digit of , which cannot be. So both end in ; reversing them shows that both start with as well. Since is divisible by and by , it is divisible by . There are four multiples of between and : , , , and . is the largest, and it is easy to see that it and its reversal, , meet all requirements. So is the answer.
Final answer
5985