Seller selling an apple for Rs.20, a Seller loses 1/6th of what it costs him. The CP of the apple is? Choose the correct choice from the following choices:  A) 10  B) 12  C) 21  D) 18  E) 20

Rationale: "sp = 20 loss = cp 21 loss = cp − sp = cp − 20 ⇒ cp 21 = cp − 20 ⇒ 20 cp 21 = 20 ⇒ cp 21 = 1 ⇒ cp = 21 c"
Answer: C