When one quantity increases, the other quantity also decreases

Like,

  If speed increases, time taken decreases

  If number workers doing a job increase, the total time to

  complete a job decreases

 

So, we can say that

     Speed and Time are inversely proportional

Similarly,

        Number of workers and total time are inversely proportional

 

How to solve questions with Inverse Proportion?

 

Let’s take an example

 

Suppose at 50 km/hr, it takes us 4 hours to reach a destination. How much time would it take to reach a destination if speed is 100 km/hr?

 

Given that,

It takes 4 hours to reach destination at speed of 50 km/hr

We need to find how much time would it take if speed is 100 km/hr

 

Let the time taken be x hours

 

Now, we draw a table

Time (in hours)

  4   x

Speed (in km/ hr )

50 100

Now, as speed increases

time taken will decreases

So, Time and Speed are in inverse proportion

 

So, we write numbers as

    x 1 y 1 = x 2 y 2

2.jpg

 

So, 

  x 1 y 1 = x 2 y 2

  4 × 50 = x  × 100

  (4 × 50)/100 = x

  4/2 = x

    2 = x

    x = 2

 

So, it takes 2 hours to travel

 

Let’s take another example

 

Suppose 5 workers complete a job in 10 days. How much time would it take if there are 10 workers?

 

Given that,

5 workers complete a job in 10 days

We need to find much time would it take if there are 10 worker

 

Let the time taken be y days

 

Now, we draw a table

Number of workers

 5   10

Time taken (days)

10    y

Now, as number of workers increases,

time taken to complete the job will decrease

So, they are in inverse proportion

 

So, we write numbers as

  x 1 y 1 = x 2 y 2

  5 × 10 = 10 × y

  (5 × 10)/10=y

    5 = y

   y = 5

 

So, it will take 5 days

  1. Chapter 13 Class 8 Direct and Inverse Proportions
  2. Concept wise

About the Author

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Davneet Singh
Davneet Singh is a graduate from Indian Institute of Technology, Kanpur. He has been teaching from the past 8 years. He provides courses for Maths and Science at Teachoo. You can check his NCERT Solutions from Class 6 to 12.