Problem: what is the total number of integers between 20 and 100 that are divisible by 9 ?
Rationale: "27 , 36 , 45 , . . . , 90,99 this is an equally spaced list ; you can use the formula : n = ( largest - smallest ) / ( ' space ' ) + 1 = ( 99 - 27 ) / ( 9 ) + 1 = 8 + 1 = 9 answer is e"
annotated_formula: add(divide(subtract(100, 20), 9), const_1)