Solve Exercise 3 on pages 139 and 140 Math 5.

Step-by-step guide to solving Exercise 3 on pages 139 and 140 Math 5

Problem:
Distance AB is 25km. On the way from A to B, a person walks 5km and then continues by car for half an hour to reach B. Calculate the speed of the car.

Solution method:
- Step 1: Calculate the car's distance traveled by subtracting pedestrian's distance from AB distance.
- Step 2: Calculate car's speed by dividing the distance the car has traveled by the time. 

Answer:
We have: Half an hour = 1/2 hour = 0.5 hours.
The car's distance traveled is:
25 - 5 = 20 (km)
The car's speed is:
20 : 0.5 = 40 (km/hour)
(or 20 : 1/2 = 40 (km/hour)
Answer: 40km/hour.

Here is the solution for Exercise 3 on page 140 Math 5, focusing on practice. Additionally, this section contains three more exercises: Solve Exercise 1 on pages 139, 140 Math 5, Solve Exercise 2 on pages 139, 140 Math 5, and Solve Exercise 4 on pages 139, 140 Math 5,check the guide Solve Math 5 pages 139, 140 to study Math 5 effectively!