Math Riddles #10 - Car Meter Riddle

Question

Today my car meter reads as 72927 kms. I notes that this is a palindrome. How many minimum kms I need to travel so my car meter find another palindrome?

Answer 1

For every choice of the three first digits of a 5-digit number, there is exactly one way to complete it so it becomes a palindrome.

The next palindrome is the one that starts with 730, namely 73037, which will be reached in $73037-72927=110$ km.

Answer 2

The number is 72927 and you want to get to the next palindrome forward. To find the next palindrome forward you want to separate your number into a left and right side so that the 2 sides have an equal number of digits like so, 72 9 27. It's fine if you have a single digit left in the middle like we do here. Next, if there is a middle number, just add +1 to it (If there was not a middle number, you'd add the +1 to the left side), and in this case we turn the 9 to a 10 so the 9 becomes a 0 and the 1 gets added to the left side because it needs to be carried over (72+1) 0 27. Now just adjust the right side appropriately to make it a palindrome, 73 0 37. The next palindrome forward is going to be at 73037.

After that, to find the difference, you subtract the smaller number from the larger number and get your difference, 73037 - 72927 = 110. You will need to travel another 110 kms to get to the next palindrome.

If you were in a magic time travelling car, that could travel backwards in time & distance, the next closest palindrome would be 72827 and would only be 100 kms away.

You can find the next palindrome of any palindrome in this way. for example,

Palindrome: 891737737198891737737198

Half it: 891737737198 891737737198

+1 left side & adjust right side: 891737737199 991737737198

Next Palindrome: 891737737199991737737198