1. Master Chen plans to use 720 square bricks with a side length of 3 meters to lay the classroom. Later, it was replaced by a square brick with a side length of 4 cents. How many pieces do I need?

What is the area of the classroom?

3× 3× 720 = 6480 square decimeter

use

6480 \( 4×4)= 405 pieces

2. When the bus goes from A to B, the speed is 60 kilometers per hour, which is 1 hour ahead of schedule. When returning, it was driving at a speed of 40 kilometers per hour, which was 1 hour later than the planned time. Find the distance between a and b.

The exact time is

(60×1+40×1) ÷ (60-40) = 5 hours.

The distance between a and b is

60× (5-1) = 240km.