Average shorcuts part-1

Average short cuts for all exams

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Average shortcuts- part-1
Time,speed and Distance shortcuts 

Basics

  • 1) There is a relationship between speed, distance and time:
           Speed   = Distance / Time OR
          Distance = Speed* Time
            where x km/hr is a speed for certain distance and y km/hr is a speed at         for same distance covered.


**** Remember that average speed is not just an average of two speeds i.e. x+y/2. It is equal to 2xy / x+y
  • 3) Always remember that during solving questions units must be same. Units can be km/hr, m/sec etc.
         **** Conversion of km/ hr to m/ sec and m/ sec to km/ hr
         x km/ hr = (x* 5/18) m/sec i.e. u just need to multiply 5/18
          Similarly, x m/sec = (x*18/5) km/sec
  • 4) As we know, Speed = Distance/ Time. Now, if in questions Distance is constant then speed will be inversely proportional to time i.e. if speed increases ,time taken will decrease and vice versa.
  For Time and Work shortcuts Click here
Examples of Average on Time and Distance
Problem 1: A man covers a distance of 600m in 2min 30sec. What will be the speed in km/hr?
Solution: Speed =Distance / Time
            ⇒ Distance covered = 600m, Time taken = 2min 30sec = 150sec
               Therefore, Speed= 600 / 150 = 4 m/sec
            ⇒ 4m/sec = (4*18/5) km/hr = 14.4 km/ hr.

Problem 2: A boy travelling from his home to school at 25 km/hr and came back at 4 km/hr. If whole journey took 5 hours 48 min. Find the distance of home and school?

Solution: In this question, distance for both speed is constant.
               ⇒ Average speed            = (2xy/ x+y) km/hr, where x and y are speeds
               ⇒ Average speed            = (2*25*4)/ 25+4 =200/29 km/hr
                  Time = 5hours 48min   = 29/5 hours
                  Now, Distance travelled = Average speed * Time
                ⇒ Distance Travelled         = (200/29)*(29/5) = 40 km
Therefore distance of school from home = 40/2 = 20km.

Problem 3: Two men start from opposite ends A and B of a linear track respectively and meet at point 60m from A. If AB= 100m. What will be the ratio of speed of both men?

Solution: According to this question, time is constant. Therefore, speed is directly proportional to distance.

Speed∝Distance
                ⇒ Ratio of distance covered by both men = 60:40 = 3:2

                ⇒ Therefore, Ratio of speeds of both men = 3:2

Problem 4: A car travels along four sides of a square at speeds of 200, 400, 600 and 800 km/hr. Find average speed?


Solution: Let x km be the side of square and y km/hr be average speed
Using basic formula, Time = Total Distance / Average Speed
     x/200 + x/400 + x/600 + x/800 = 4x/y ⇒ 25x/ 2400 = 4x/ y⇒ 
                                                  y= 384

                            ⇒ Average speed = 384 km/hr