Triathlete Dan runs along a 4-mile stretch of river and then swims back along the same route. If Dan runs at a rate of 10 miles per hour and swims at a rate of 6 miles per hour, what is his average rate for the entire trip in miles per minute? Answers
A) 1 / 8  B) 2 / 15  C) 3 / 15  D) 1 / 4  E) 3 / 8

Rationale: "dan travels 8 miles round trip . running part : ( 4 / 10 * 60 = 24 minutes ) swimming part : ( 4 / 6 * 60 = 40 minutes ) 8 miles in ( 24 + 40 ) minutes 8 / 64 = 1 / 8 mile per minute answer : 1 / 8 mile per minute"
Answer: A