The present worth of a certain bill due sometime hence is Rs. 400 and the true discount is Rs. 20. What is the banker's discount?	Choose one.
A) rs . 19  B) rs . 22  C) rs . 20  D) rs . 21  E) none of these

Rationale: "explanation : bg = ( td ) 2 / pw = 202 / 400 = rs . 1 bg = bd – td = > 1 = bd - 20 = > bd = 1 + 20 = rs . 21 answer : option d"
Answer: D