Problem: if the average ( arithmetic mean ) of a and b is 100 , and the average of b and c is 160 , what is the value of a − c ?
Rationale: "question : a - c = ? ( a + b ) / 2 = 100 = = = > a + b = 200 ( b + c ) / 2 = 160 = = = > b + c = 320 ( a + b ) - ( b + c ) = 200 - 320 = = = > a + b - b - c = - 120 = = = > a - c = - 120 answer : a"
annotated_formula: subtract(multiply(160, const_2), multiply(100, const_2))