Theory:

Inverse proportion: As one amount increases, another amount decreases at the same rate or as one amount decreases, another amount increases at the same rate is known as inverse proportion.
The symbol for "Inverse proportion" is 1α
Example:
Consider a situation that you're going to your native place in a vehicle. Your destination is \(360 km\) far from your starting point. Let's see a few cases to understand inverse proportion.
 
Case 1)
If you travel at the speed of \(120 kmpl\), you will reach your destination in \(3 hours\).
 
Case 2)
If you travel at the speed of \(90 kmpl\), you will reach your destination in \(4 hours\).
 
Case 3)
If you travel at the speed of \(60 kmpl\), you will reach your destination in \(6 hours\).
 
Case 4)
If you travel at the speed of \(40 kmpl\), you will reach your destination in \(9 hours\).
 
Case 5)
If you travel at the speed of \(30 kmpl\), you will reach your destination in \(12 hours\).
From the above cases, we find that time taken to reach your destination is decreasing when the speed of the vehicle is increasing.   
This could be written as
 
Speed1Time
 
Therefore speed and time are in inverse proportion.