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The cost of pure ghee is Rs x per kg. If the price of it is increased by 20 per kg, how many kilograms of pure ghee does one get in Rs 800 according to the new price?
(a) $\dfrac{800}{x+20}$
(b) $\dfrac{800}{x-20}$
(c) $\dfrac{x+20}{800}$
(d) $\dfrac{800}{x}$

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Hint: To solve this problem, we should be aware about the basics of unitary method. We will try to find the amount of pure ghee we can get in Rs 1 and then multiply that by 800 to get the required amount of pure ghee.

Complete step by step answer:
Now, we can use the unitary method to solve the given problem in hand. Basically, the unitary method is a technique for solving a problem by first finding the value of a single unit, and then finding the necessary value by multiplying the single unit value. In essence, this method is used to find the value of a unit from the value of a multiple, and hence the value of a multiple. To explain this definition,

Let’s say, 2 bags cost 50 rupees and suppose we want to know how many bags we can buy from 75 rupees. What we do is, we see how many bags can be bought for 1 rupee. Then we multiply that by 75. Thus,

For 50 rupees, we have 2 bags

For 1 rupee, we have $\dfrac{1}{25}$bags

For 75 rupees, we have $\dfrac{75}{25}$=3 bags

We use a similar methodology to solve the given problem in hand. Thus, we have,

We are given that the price is increased by Rs 20 per kg. Thus, the new price is Rs (x+20) per kg. We have,

Cost of pure ghee is Rs x+20 per kg.

Thus, in Rs 1, we would have $\dfrac{1}{x+20}$kg of pure ghee.

Now, to find the amount of pure ghee for Rs 800, we have $\dfrac{800}{x+20}$kg of pure ghee.

Hence, the correct answer is (a) $\dfrac{800}{x+20}$.


Hint: The use of unitary method is only applicable when the quantities are directly related to each other. In case a quantity is directly related to square/cube/inverse or any other operations, the unitary method yields inaccurate results. For example, if x varies as a square of y, we cannot use a unitary method between x and y variables.
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