20. A palindrome number is the same either read from left to right or right to left, for example. 121 is a palindrome number. How many 5 -digit palindrome numbers are there together?
20 Ans: 900 .
Such a palindrome number is completely determined by its first three digits. There are $9,10,10$ choices of the 1 st, 2 nd and 3 rd digits respectively, hence total $=900$.