City A and City B are 140 miles apart. Train C departs City A, heading towards City B, at 4:00 and travels at 40 miles per hour. Train D departs City B, heading towards City A, at 4:40 and travels at 20 miles per hour. The trains travel on parallel tracks. At what time do the two trains meet? Choose the correct option
A) 5 : 00  B) 5 : 30  C) 6 : 40  D) 6 : 30  E) 7 : 00

Rationale: "train c has traveled 20 mi in the half hour before train d has started its journey . 140 - 20 = 120 40 + 20 = 60 mph 120 mi / 60 mph = 2 hrs 4 : 40 pm + 2 hrs = 6 : 40 pm answer : c . 6 : 40"
Answer: C