Meera walked to her school at a speed of 3 miles per hour. Once she reached the school, she realized that she had forgotten to bring her books, so she rushed back home at a speed of 6 miles per hour. She then walked back to the school at a speed of 4 miles per hour. She took the same route on each occasion.

Quantity I: her average speed over the entire journey
Quantity II: 4 mph