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Last updated on June 11th, 2025

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Inverse Proportion

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The relationship between the two variables in mathematics is defined using proportions. It is of two types: direct or inverse proportion. Inverse proportion is when the value of one variable decreases and the other increases. Let’s explore more about this connection.

Inverse Proportion for Indian Students
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What is Inversely Proportional in Math?

In mathematics, the two commonly used proportions are direct proportion and inverse proportion. Inverse proportion is the opposite of direct proportion.

 

Two values are in inverse proportion when one decreases as the other increases. If the product of the two quantities equals a constant value, regardless of the change in their values, they are considered to be in inverse proportion. 
 

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Inversely Proportional Formula

The mathematical representation of the inversely proportional relationship of two quantities is given below:

  • x and y are two inversely proportional variables, and the product always remains constant(k).

            x × y = k

  • The commonly used formula that indicates y is inversely proportional to x:

            y = k/x

  • When two sets of values are inversely proportional, their product remains constant.

           x1 × y1 = x2 × y2

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Difference Between Direct and Inverse Proportionality

Direct and inverse proportionality are mathematical tools that measure the change in one quantity with the change in the other. The key differences between the two are listed below:
 

Direct Proportionality Inverse Proportionality
Two values increase or decrease proportionally. If one quantity increases, the other decreases.
Formula : y = kx Formula: x  y = k
The graph gives a straight line. The graph gives a curved line.
For example: The more you buy the more you pay For example: The more you eat, the less is on your plate.

 

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Tips and Tricks for Inversely Proportional

Learning about inversely proportional relationships helps children understand the mathematical connection between quantities. Let’s look at a few tips and tricks to understand this connection easily:
 

  • Students should always remember that when one quantity increases, the other decreases, or vice versa then it is inverse proportion .

 

  • The product of two inversely proportional quantities will always result in the constant. i.e., k = x 1 ×  y1.

 

  • To learn inverse proportionality try to use it in real-life situations. That is where one quantity increases as the other decreases.  For example: The more the number of people, the less time it will take.

 

  • The graph of inverse proportion gives a curved line. i.e., as x goes up, y comes down.

 

  • You can imagine a see-saw to better understand inverse proportionality. i.e., when one rises, the other falls.
     
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Real-World Applications of Inversely Proportional

Inversely proportional relationships can be observed in many real-life situations. Understanding their real-world applications helps students apply the concept quickly. Let’s learn how it can be applied:

  • Students can easily apply the concept in situations where an increase in one quantity causes a decrease in the other. For example: the number of people working on a task and its completion time.

 

  • Understanding this relationship helps students quickly learn scientific theories or laws where the quantities are inversely connected. For example, Boyle’s Law states that pressure is inversely proportional to the volume (pressure ∝ 1/volume).

 

  • They will be able to understand the facts, such as why doctors prescribe highly concentrated medicines in lower doses.

 

  • Scientists and chemists utilize the concept to predict how the intensity of one quantity affects another in their inventions or experiments.

 

  • Moreover, we unknowingly apply this concept while cooking. For example, increasing the intensity of heat to decrease the cooking time.
     
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Common Mistakes and How to Avoid Them in Inversely Proportional

Learning inverse proportionality is easy once you understand what it is. However, students tend to make mistakes. Let’s look at a few common errors and the ways to avoid them
 

Mistake 1

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Confusion between Inverse Proportion and Direct Proportion

Students often think that when one quantity increases, the other will also increase.
They don’t understand that in inverse proportion, the opposite occurs.
 

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To grasp the inverse proportion easily, think of how the see-saw works, when one rises the other falls, or vice versa.

Mistake 2

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 Ignoring Units

Not checking whether the final result includes a unit can lead to a lack of clarity. For example: km/h, miles, etc.
 

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Ensure that you specify the appropriate units at each step of the calculation to avoid confusion.
 

Mistake 3

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Assuming a linear graph

Students used to think that the inverse proportion graph is also a linear graph but it is incorrect.
 

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Always remember that in direct proportion, the graph is a straight line, whereas in inverse proportion, the graph is a curved line (hyperbola).

Mistake 4

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Not Finding the Constant k First

They attempt to determine the unknown value before calculating the value of k.

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They need to find the value of k first and then use it to determine the unknown value.
 

Mistake 5

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Using Incorrect Formula
Some students mistakenly use the direct proportion formula (y = kx), for problems that involve inverse proportion.
 

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It can be resolved by understanding the right formula to use. For inverse proportion, use the formula: x1 × y1 = x2 × y2.
 

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Solved Examples of Inversely Proportional

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Problem 1

If 2 students can complete a task in 10 days. How many days will it take if 5 students are assigned the same task?

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It requires 4 days for 5 students to finish the task.
 

Explanation

To find the number of days required to complete the task we use the formula, 

x1 × y1 = x2 × y2

Here, x1 = 2

x2 = 5 

y1 = 10

y2 = x1 × y1 / x2

y2 = 2 × 10 / 5 

= 20 / 5 = 4

So, it requires 4 days for 5 students to finish the task.
 

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Problem 2

A factory uses 6 machines to produce a batch of items in 9 hours. If 12 machines are used, how long will it take?

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 It will take 4.5 hours to produce the batch.
 

Explanation

Here, we use the formula: k = x × y

k  = 6 × 9 = 54

Now, we determine the value of y when x = 12

y = 54/12 = 4.5

So, using 12 machines, it will take 4.5 hours to produce the batch.

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Problem 3

If 5 taps fill a water tank in 40 minutes, how long will it take if 8 taps are used?

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It will take 25 minutes to fill a water tank with 8 taps.
 

Explanation

Since, the number of taps ∝ 1/ time taken, we use the formula:

x1 × y1 = x2 × y2

5 × 40 = 8 × y

200 = 8y

Y = 200/8 = 25

So, it will require 25 minutes to fill a water tank with 8 taps. 

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Problem 4

A gas has a pressure of 150 kPa and a volume of 3 liters. If the pressure increases to 300 kPa, what will be the new volume?

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The new volume is 1.5 liters.
 

Explanation

Use the formula for inverse proportion:

x1 × y1 = x2 × y2

150 × 3 = 300 × y

 450 = 300 y

 y = 450/300 = 1.5

Therefore, the new volume is 1.5 liters.

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Problem 5

A sound wave has a frequency of 400 Hz and a wavelength of 2 m. If the frequency is increased to 800 Hz, what will be the new wavelength?

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The new wavelength is 1 meter.
 

Explanation

We use the formula for inverse proportion as frequency and wavelength:

x1 × y1 = x2 × y2

400 × 2 = 800 × y

800 = 800y

y = 800/800 = 1

Therefore, the new wavelength is 1 meter.
 

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FAQs on Inverse Proportional

1.What do you mean by an inversely proportional relationship?

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2.Give the formula for inversely proportional.

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3.How is inversely proportional different from directly proportional?

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4.Cite an example of inversely proportional.

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5.How can we solve an inverse proportion problem?

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Dr. Sarita Ghanshyam Tiwari

About the Author

Dr. Sarita Tiwari is a passionate educator specializing in Commercial Math, Vedic Math, and Abacus, with a mission to make numbers magical for young learners. With 8+ years of teaching experience and a Ph.D. in Business Economics, she blends academic rigo

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