Speed, Time and Distance
Speed
The distance travelled by any object or a person in unit time is known as speed of the object or of the person.
Speed =  Distance travelled  
Time taken 
The unit of speed is m/s or km/h.
Time
The duration in hours, minutes or seconds spent to cover a certain distance is called the time.
Time =  Distance  
Speed 
Distance
The length of the path travelled by any object or a person between two places is known as distance. The unit of distance is m or km.
Distance = (Speed x Time).
For example : Nikunj covered a distance of 75 km between Meerut and Delhi in 3 h.
∴ Distance travelled in 1 h = 75/3 = 25km
Hence, speed of Nikunj = 25 km/h
Magical Tricks
Trick #1: To convert the speed of an object from km/h to m/s, multiply the speed by 5/18.
x km/hr =  x x  5  m/sec.  
18 
For example:
54 km/hr =  54 x  5  m/sec = 15m/s  
18 
Trick #2: To convert the speed of an object from m/s to km/h, multiply the speed by 18/5.
x m/sec =  x x  18  km/hr.  
5 
For example
25 m/sec =  25 x  18  = 90 km/hr.  
5 
Important Facts & Formulae

Speed, Time and Distance:
Speed = Distance , Time = Distance , Time Speed Distance = (Speed x Time).

km/hr to m/sec conversion:
x km/hr = x x 5 m/sec. 18 
m/sec to km/hr conversion:
x m/sec = x x 18 km/hr. 5 
If the ratio of the speeds of A and B is a : b. Then,
the times taken by then to cover the same distance is
1 : 1 or b : a. a b 
Suppose a man covers a certain distance at x km/hr and an equal distance at y km/hr. Then,
the average speed during the whole journey is
2xy km/hr. x + y
TIPS on cracking Aptitude Questions on Speed, Time and Distance
Tip #1: Use the formula Speed = Distance/ time while ensuring that units are same
Question: In a flight of 600 km, an aircraft was slowed down due to bad weather. Its average speed for the trip was reduced by 200 km/hr and the time of flight increased by 30 minutes. Calculate the duration of the flight.
Solution:
Let the average duration of the flight be ‘t’ hours. Distance = 600 km.
Average Speed (km/ hr) = 600/ t
Average Speed – 200 = 600 / (t + 0.5)
(600/ t) – 200 = 600 / (t + 0.5)
(600 – 200 t) (t + 0.5) = 600 t
=> 2t^{2} + t  3 = 0 =>t = 1 hr
Duration of the flight = 1.5 hrs
Question: Robert is travelling on his cycle and has calculated to reach point A at 2 P.M. if he travels at 10 km/hr. But he will reach there at 12 noon if he travels at 15 km/hr. At what speed must he travel to reach A at 1 P.M.?
Solution:
According to the question,
(D / 10) – (D / 15) = 2 =>D = 60 km.
Time taken to reach A at 10 km/hr = 60 / 10 = 6 hours. (Starting time is 8 AM)
Therefore, we need to find the speed to cover 60 km should be in 5 hours.
Hence, speed = 60 / 5 =12 km/hr.
Tip #2: If a person travels same distances with different speeds, then the average speed is not the arithmetic mean but the harmonic mean
If a person covers a distance d first at x km/hr and then covers the same distance d at y km/hr, then the average speed is:
= Total distance travelled/ Total time taken
= 2d/ (d/x + d/y)
= 2d/ [(yd + xd)/xy]
= 2dxy/[d(x+y)]
= 2xy/ x+y (Harmonic mean of x and y)
Question: A travels 25km at 50 km/hr and then 25km again with 70km/hr. What is A’s average speed during the whole journey?
Solution:
Average speed for the whole journey = (2x50x70) / (50+70) = 58.3km/hr
Tip #3: If A runs x times faster than B, A’s speed is actually 1+x the speed of B
Question: A runs 1? times as fast as B. If A gives B a start of 80 m, how far must the winning post be so that A and B might reach it at the same time?
Solution:
Speed of A, Sa = 5/3 x Sb
Let the distance of the course be ‘d’ meters
Time taken by A to cover distance ‘d’ = Time taken by B to cover distance ‘d80’
d/[5/3 x Sb] = (d80)/Sb
3d = 5d – 400
=> 2d = 640 => d = 200m
Question: A runs 1? times faster than B. If A gives B a start of 80 m, how far must the winning post be so that A and B might reach it at the same time?
Solution:
Speed of A, Sa = (1 + 5/3) x Sb = 8/3 x Sb
Let the distance of the course be ‘d’ meters
Time taken by A to cover distance ‘d’ = Time taken by B to cover distance ‘d80’
d/[8/3 x Sb] = (d80)/Sb
3d = 8d – 640
=> 5d = 640 => d = 128m
[Note: Here, A 5/3 times faster than B, i.e., A’s speed = B’s speed + 5/3 times B’s speed = 8/3 times B’s speed.]