Q: Jeff drives at the speed of 50 km/h from destination A to B. Jackson, who departs 30 minutes after Jeff, drives at the speed of 70 km/h from destination A to B. Both Jeff and Jackson reach destination B at the same time. What is the distance between destination A and destination B?
A: At first glance, the students may question whether there is an answer. Although the speed and the time are given, the time is not the time taken to travel from destination A to destination B.
No worry. The distance travelled is the same for Jeff and Jackson and we can use this information to solve the question. Please note that the unit for time is minute, converting to hour, it is 0.5 hour.
Let’s take t = time take for Jeff to reach destination B.
The distance travelled for Jeff = speed x time
= 50 x t
The distance travelled for Jackson = speed x time
= 70 x (t – 0.5)
The distance travelled is the same,
50t = 70 (t – 0.5)
Solving the above equation,
50t = 70t – 35
t = 35 ÷ 20
= 1.75 h
From the speed triangle, distance (travelled by Jeff) = speed x time
Distance = 50 x 1.75
= 87.5 km
Checking the answer
The distance from destination A to destination B is the same. Thus, the distance travelled by Jeff and Jackson should be the same.
Distance (travelled by Jackson) = speed x time,
Distance = 70 x 1.25
= 87.5 km
1. Look for all the necessary information in the question and use all of them.
2. Speed is a simple concept. Thus, questions about speed are normally combined with other concepts, for example, circles. Other than the speed formula and the unit conversion, the students must be prepared to use other concepts to solve speed questions.