If 10 typists can type 20 letters in 20 minutes, then how many letters will 40 typists working at the same rate complete in 1 hour?	Choose the correct answer:  A) 220  B) 210  C) 200  D) 240  E) 260

Rationale: "no . of letters typing by 10 typists in 20 minutes = 20 no . of letters typing by 10 typists in 60 minutes = 20 * 3 = 60 no . of letters typing by 40 typists in 60 minutes = 60 / 10 * 40 = 240 answer : d"
Answer: D