May 27, 2011
An “upside up” number is a number that reads the same when it is rotated 180°. For instance, 689 and 1961 are upside up numbers.
Your task is to find the next upside up number greater than 1961, and to count the number of upside up numbers less than ten thousand. When you are finished, you are welcome to read or run a suggested solution, or to post your own solution or discuss the exercise in the comments below.