My car has a 5-digit odometer, which measures the miles since the car was built, and a 3-digit trip meter, which measures the miles since I last set it. Every so often, one or both of the readings is a palindrome. The meters reset to 000 after 999 and to 00000 after 99999.

The current readings are 123 and 12345. Assuming that I do not reset the trip meter, when is the next time both readings will be palindromes?

When was the most recent time both readings were palindromes?

Prove no matter what the mileage and trip meters read, they can eventually be made to both be palindromes without resetting the trip meter.

Note: A palindrome reads the same forwards and backwards, like 262 or 37173.

The trip odometer cycles in a period of 1000 miles.

Whatever the trip odometer shows at some time it will get to be a palindrome.  At that time the main odometer will have some value, with digits a,b,c,d,e, and probably not palindromic.

However, we can go a multiple of 1000 miles (therefore returning the odometer to its palindromic state) to advance the main odometer to e,d,c,d,e.  This will be after 1000*(10*e+d-10*a-b) miles, or if this is negative, add 100,000 more.

Edited on June 16, 2004, 10:11 am

Posted by Charlie on 2004-06-16 10:08:07