Problem: a palindrome is a number that reads the same forward and backward , such as 616 . how many even , 4 - digit numbers are palindromes ?
Rationale: "first recognize you only need to consider the first two digits ( because the second two are just the first two flipped ) there are 90 possibilities for the first two digits of a 4 digit number , 10 - 99 inclusive . everything starting with a 2,4 , 6,8 will be odd , which is 4 / 9 ths of the combinations . 4 / 9 * 90 = 40 answer : a"
annotated_formula: divide(power(const_10, divide(4, const_2)), const_2)