Problem: what least number should be added to 1536 , so that the sum is completely divisible by 21 ?
Rationale: "1536 ÷ 21 = 73 reminder - 3 3 + 18 = 21 hence 18 should be added to 1536 so that the sum will be divisible by 21 answer : c"
annotated_formula: subtract(21, reminder(1536, 21))