Quote
When Anna gets home after a long day at the university, she decides to ride her bicycle. She begins biking right outside her front door. First she rides on a level road, then she comes to a hill and bikes to the top. When she gets to the top of the hill, she turns around and she bikes back exactly the way she came. Now, on lever ground, Anna can bike at 20 miles an hour. Uphill, 15 miles an hour. And downhill, 30 miles an hour. Upon her return home, she notices that she had biked for exactly two hours. How far did Anna rider on her bicycle?
This funny problem gets a solution once you called A the length (in miles) of the flat road, and B the length (in miles) of the hill.
2 hours = A/20 * 2 + B/15 + B/30
which simplifies to :
2 = (A+B) / 10
And notice that you are just asked about A+B, which is 20 miles.