Important notes and formula

We say two quantities are directly proportional if on increasing or decreasing one of the quantity also results in increase or decrease of the other quantity.

Example:

Price of Apple is directly proportional to the number of unit bought.

Price of 1kg Apple is Rs. 100

Then price of 5kg Apple = 5 x 100 = Rs. 500

Example:

If the price tag of Php for Beginners book is $5.99 then what is the cost of 3 units?

Cost of 3 books = 3 x 5.99

= $17.97

We say two quantities are indirectly proportional if on increasing or decreasing one of the quantity results in decrease or increase of the other quantity.

If there are more worker then the same work will be done in less time.

So, number of worker and time taken to complete a task are inversely or indirectly proportional.

More works imples Less time

Similarly, value of a car decrease (depreciates) as time increases. So, value of car is inversely proportional to time.

Example:

If the cost of Audi is Rs. 28,00,000 when it was bought. If the rate of depreciation is 2% per annum. Then what is the cost of the car after 1 year?

Current cost = Rs. 28,00,000

Depreciation = 2%

Depreciated value = Rs. (28,00,000 x 2%)

= Rs. 56,000

Cost of the car after 1 year = Rs. (28,00,000 - 56,000)

= Rs. 27,44,000

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