Problem: A man makes a trip by automobile at an average speed of 50 mph. He returns over the same route at an average speed of 45 mph. His average speed for the entire trip is:
Answer Choices:
A. 47197​
B. 4741​
C. 4721​
D. 471911​
E. none of these
Solution:
A1​=21​(501​+451​);∴A=47197​.