Problem: a and b can do a piece of work in 11 days . with the help of c they finish the work in 5 days . c alone can do that piece of work in ?
Rationale: "c = 1 / 5 – 1 / 11 = 6 / 55 = > 9.2 days answer : d"
annotated_formula: inverse(subtract(5, divide(5, 11)))