Think of the place around which the whole number must be centered is the hundreds place. If you keep adding 1 to the given palindrome, the digits to the right of the hundreds place cannot match 50. The next number that gives a matching 05 to the right of the hundreds place is 50905. If you start adding 1 to this number, you’ll eventually get 51000. Now the digits you need to match on the other side are 5 1. So add 15 and get 51015. Subtract 51015 – 50905 = 110 (D).