Doubt in Profit & Loss solved example from Totalgadha.

MBA CAT 2023
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In the Totalgadha’s Profit & Loss lesson, there is this example problem.Q: A shopkeeper buys oranges at the rate of 4 for Rs20 and sells them at rate of 5 for Rs30. What is his profit percentage? Answer: To find profit percentage we will ha…

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In the Totalgadha's Profit & Loss lesson, there is this example problem.

Q: A shopkeeper buys oranges at the rate of 4 for Rs20 and sells them at rate of 5 for Rs30. What is his profit percentage?

Answer: To find profit percentage we will have to first make numbers of oranges bought and sold equal. Therefore, we take LCM of 4 and 5, i.e 20. The shopkeeper buys 20 oranges for Rs.100 and sells 20 oranges for Rs.120.

Therefore his profit percentage=profit x 100/CP => 20 x 100/100 = 20%.

My doubt is, how did we decide that shopkeeper buys 20 oranges for 100 rupees?? and then he sold it for 120, so is the LCM added to that previously CP 100?? Why so?? What is LCM doing here??
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in norway, when the price of eggs dropped by 20%, 20 more eggs could be obtained for 20 kroner. what was the price of one egg earlier?

please provide solutions for this question...