Problem: if you write down all the numbers from 1 to 100 , then how many times do you write 1 ?
Rationale: "explanation : explanation : clearly , from 1 to 100 , there are ten numbers with 1 as the unit ' s digit - 1 , 11 , 21 , 31 , 41 , 51 , 61 , 71 , 81 , 91 ; and ten numbers with 1 as the ten ' s digit - 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 , 19 , 100 so , required number = 10 + 11 = 21 . answer : d"
annotated_formula: divide(subtract(100, 1), 1)