Problem: how many integers k greater than 100 and less than 600 are there such that if the hundreds and the unit digits of k are reversed , the resulting integer is k + 99 ?
Rationale: "not sure if this is the shortest . . but this is how i did this there are 8 sets of integers with hundreds and units digits exchanged that satisfies k + 99 . 1 . 102 | 201 ( satisfies k + 99 , where k = 102 ) 2 . 203 | 302 ( satisfies k + 99 , where k = 203 ) 3 . . . . 4 . 405 | 504 each set has 10 such numbers . 1 . 102 | 201 ( still k + 99 holds good ) 2 . 112 | 211 3 . 122 | 221 4 . 132 | 231 5 . . . . 6 . . . . 7 . . . . 8 . . . . 9 . 182 | 281 10 . 192 | 291 therefore , 4 sets with 10 such number in each set will give 4 x 10 = 40 integers . a"
annotated_formula: multiply(const_10, subtract(const_10, const_2))