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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