Theory:

We see several situations in our daily life, in which we have to see the variation in one quantity which changes in the other.
 
The two quantities \(X\) and \(Y\) is said to be in proportion if they increase (or decrease) shows how they are related to each other in quantities and amount.
 
The quantity that changes the amount in relation to each other is governed by the rules of proportion.
 
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Example:
(i) If the number of items purchased increases, the total cost would also increase.
 
(ii) The more money deposited in a bank, the more interest earned.
 
(iii) As the speed of the vehicle increases, the amount of time taken to cover the same distance decreases.
 
(iv) For a given job, the more the number of workers, the less time will take to complete the work.