Page 125 - ICSE Math 6

Example 4: Anant can run 250 metres in 50 seconds. Find:
(a) his speed

(b) distance covered by him in 10 seconds
1
(c) time taken by him to cover km
2
Distance 250 m
Solution: (a) Speed = = = 5 m s –1
Time 50 s
–1
(b) Distance = Speed × Time = 5 m s × 10 s = 50 m
1 1000
(c) Distance = km = m = 500 m
2 2
Distance 500 m
Time = = = 100 s
Speed 5 m s –1

–1
Example 5: A motorist covers first 50 km of his journey at a speed of 40 km h and the remaining
–1
70 km at a speed of 60 km h . Find:
(a) the total time taken to complete the journey
(b) his average speed
50 km  Distance 
Solution: (a) Time taken to cover first 50 km = –1  Time = 
40 km h  Speed 
5
= h
4
7
Time taken to cover the remaining 70 km = 70 km –1 = h
60 km h 6
5 7 15 + 14 29 5
\ Total time taken = h + h = h = h = 2 h
4 6 12 12 12
50 km + 70 km
(b) Average speed = Maths Info
29/12 h

120 × 12 Average speed = Total distance covered
= km h –1 Total time taken
29
19
= 49 km h –1
29
–1
–1
Example 6: Two athletes X and Y start running from the same point with speeds 10 km h and 12 km h
respectively. Find the distance between them after 3 hours, if they run in:
(a) the same direction (b) opposite directions
Solution: (a) Distance covered by X in 3 hours = Speed × Time
–1
= 10 km h × 3 h = 30 km
Distance covered by Y in 3 hours = Speed × Time
–1
= 12 km h × 3 h = 36 km
\ Required distance = Difference between the distances covered by X and Y
= 36 km – 30 km = 6 km
(b) Required distance = Sum of the distances covered by X and Y

= 30 km + 36 km = 66 km

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