Page 129 - ICSE Math 7

Example 1: A truck covers a distance of 60 km in 45 minutes. Determine:
–1
(a) its speed in km h .
–1
(b) its speed in m s .
(c) time taken by the truck to cover 960 km.
(d) distance covered by the truck in 27 min.

45 3
Solution: (a) Distance = 60 km, time = 45 min = 60 h = h
4
60
–1
Speed = km h = 80 km h –1
3/4
5
–1
–1
(b) Speed in m s = Speed in km h × 18
5
= 80 × m s –1
18
2
= 22 m s –1
9
(c) Time taken to cover 960 km = Distance
Speed
= 960 h = 12 h
80
(d) Distance covered in 27 min = Speed × Time

= 80 × 27 = 36 km
60
Example 2: The distance between two stations A and B can be covered in 3 h with a speed of
–1
–1
45 km h . How much time will be saved if the speed is increased by 9 km h ?
Solution: Distance between the two stations = Speed × Time
= (45 × 3) km = 135 km
–1
Increased speed = (45 + 9) km h = 54 km h –1
Distance 135
Time required to cover the distance with increased speed = Speed = 54 h

135 27 1
\ Time saved = 3 – h = h = h
54 54 2
1
Example 3: A car covered 225 km in 3 hours. If one-third of the distance was covered at a speed
4
–1
of 75 km h , at what speed was the remaining distance covered?
1
Solution: One-third of the total distance = × 225 km = 75 km
3
Speed = 75 km h –1
Distance 75
Time taken = Speed = 75 h = 1 h

Remaining distance to be covered = (225 – 75) km = 150 km

1 1 9
Time taken to cover 150 km = 3 – 1 h = 2 h = h
4 4 4
150 150 × 4 2
–1
–1
–1
\ Speed for the remaining journey = 9/4 km h = 9 km h = 66 km h
3
= 66.67 km h –1
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