Difference between SI and CI

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Time and work (method - 2)

 We solve time and work problems also by using time and efficiency. So now, we see hoe to solve in this method.

               

	A	B
Time T
Efficiency E

The table is the basic rule. Now when we fill the columns in table, Ta * Ea  = Tb * Eb

ie., efficiency should be appropriate. Here Efficiency is nothing but the ratio in reverse order.

Efficiencies should be noted in reverse order..  of ratios.

If Ratios of A and B are 2:3, then efficiencies must be.. 3:2… Now Ea=3 and Eb=2

Now,  work done is..W = T x E (of  either A or B, as both are same)

Work done by A = W/Ea                 ;(Ea=Efficiency of A)

Work done by B = W/Eb                  ; (Eb=Efficiency of B)

Work done by A+B = W/E(a+b)     ; (Ea+b=Sum of Efficiencies of A and B)

Example:

A can do a work in 10 days. B can complete the same work in 20 days. If they work together, how many days they take to complete.

Ans : 6 2/3 days

Solution:

    A   B

T  10 20                                                                                                 w = 20

E   2   1 (as Ratio of A and B but in reverse order…)            Wa+b = 20/3 = 6 2/3 days…

 Refer Method - 1 here..

Time, Distance and Train Problems

This is the major topic in any competitive examinations. So, I am now explaining the shortcut and the fastest method for solving these problems. There are four different types of problems in this topic.
They are:          1. Problems on Time taken by train

2. Problems on Distance covered by train

3. Problems on speed of train

4. Problems on 2 trains in same direction

5. Problems on 2 trains in opposite direction

6. Problems on finding Length of train or platform

7. Problems on platform crossing time

8. Average speeds

  You all know this basic Triangle.

/\

/  D \

/ T |  S \

D – Distance , T – Time,  S – Speed , L – length of train, Lp – Platform length

 Where d=t *s

                T= d/s

                S= d/t

Time calculated when distance and speed is given          :               t=d/s

                                When length (l) and speed is given         :        t= l/s      (pole/man crossing time)

Time taken to cross platform     :               (L+Lp)/s

Relative speeds                               : Same direction = (S1 – S2)         ; (where s1 >s2)

                                                : Opposite direction = (S1+S2)

Time taken by trains cross in same direction is (sum of lengths of trains / diff in speeds)

Time at which 2 trains pass by is (Sum of lengths / sum of speeds)

Important NOTE:

Conversions:                     18 kmph = 5mps

                10 mts --->1kmph ---> 36 sec

                                20 mts ---> 2kmph ---> 36 sec

                                30 mts ---> 3kmph ---> 36 sec and so on..

Ie, a vehicle travels 10 meters distance with 1kmph speed in 36 seconds

Vehicle travels 20 meters distance with 2kmph speed in 36 seconds

Vehicle travels 30 meters distance with 3kmph speed in 36 seconds

Vehicle travels 40 meters distance with 4kmph speed in 36 seconds and so on..

Now, we will see how to use the above shortcut.

A car travels with a speed of 108kmph. What distance will be covered in 15 sec?

Ans : 450mts

Solution: Write the given parameters in above format

1080mts --> 108 kmph ---> 36sec

Now we know distance travelled in 36 sec. Find 15sec distance using this.

Ie by Ratio method as i already mentioned in my previous posts Ratio and Proportion

                36 --> 1080

                15 --> ??                (450)

Average speeds:

When distance is constant ie speeds are given  :               2ab/ (a+b)

When distances and times are given                  :               Total distance/ Total time

Try some problems and Answer them:

                         

1.90kmph in m/s

2.52m/s in kmph

3.A vehicle covers a distance of 600mts in 2 mins. What is Speed in kmph

4.A car runs at 78kmph. What is the distance covered in 1min 12 sec?

5.At what time car travels over a distance of 420mts, if it runs at 90kmph?

6.The ratio between two trains’ speeds is 7:8. If second train runs 400kmph in 5hrs, the speed of first train is?

7.Speeds of A and B are in ration 3:4. A takes 20 mins more than B to reach the destination. Then What is time taken by A and B to reach the destination?

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