Here we will learn to solve different types of problems on calculating speed.
We know, the speed of a moving body is the distance traveled by it in unit time.
Formula to find out speed = distance/time
Word problems on calculating speed:
1. A man walks 20 km in 4 hours. Find his speed.
Solution:
Distance covered = 20 km
Time taken = 4 hours
We know, speed = distance/time
= 20/4 km/hr
Therefore, speed = 5 km/hr
2. A car covers a distance of 450 m in 1 minute whereas a train covers 69 km in 45 minutes. Find the ratio of their speeds.
Solution:
Speed of car = Distance covered/Time taken = 450/60 m/sec = 15/2
= 15/2 × 18/5 km/hr
= 27 km/hr
Distance covered by train = 69 km
Time taken = 45 min = 45/60 hr = 3/4 hr
Therefore, speed of trains = 69/(3/4) km/hr
= 69/1 × 4/3 km/hr
= 92 km/hr
Therefore, ratio of their speed i.e., speed of car/speed of train = 27/92 = 27 : 92
3. Kate travels a distance of 9 km from her house to the school by auto-rickshaw at 18 km/hr and returns on rickshaw at 15 km/hr. Find the average speed for the whole journey.
Solution:
Time taken by Kate to reach school = distance/speed = 9/18 hr = 1/2 hr
Time taken by Kate to reach house to school = 9/15 = 3/5 hr
Total time of journey = (1/2 + 3/5) hr
Total time of journey = (5 + 6)/10 = 11/10 hr
Total distance covered = (9 + 9) km = 18 km
Therefore, average speed for the whole journey = distance/speed = 18/(11/10) km/hr
= 18/1 × 10/11 = (18 × 10)/(1 × 11) km/hr
= 180/11 km/hr
= 16.3 km/hr (approximately)
Relationship between Speed, Distance and Time
Problems on Calculating Distance
Two Objects Move in Same Direction
Two Objects Move in Opposite Direction
Train Passes a Moving Object in the Same Direction
Train Passes a Moving Object in the Opposite Direction
Two Trains Passes in the Same Direction
Two Trains Passes in the Opposite Direction
8th Grade Math Practice
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