Question
With the decrease in prices of tea by $15\%$ Tonu, the chaiwallah, was able to buy $2\ kg$ more of tea with the same $Rs. 45$ that he spent each month on buying tea leaves for his chai shop. What was the reduced price of tea? What was the original price of tea?

Answer

Let chaiwallah purchase $y$ kg tea.
Let price of tea per kg $= Rs. x$
Discount $15\%$ per kg$=\text{x}-\frac{15}{100}\times\text{x}=\frac{85\text{x}}{100}$
Chailwallah can buy $2\ kg$ extra with $15\%$ discount.
But without discount,$xy = 45$
$\Big(\text{x}-\frac{15}{100}\Big)\times\text{x}=\Big(\frac{85\text{x}}{100}\Big)(\text{x+2})=45$
After solving Eqs. $(i)$ and $(ii)$
$\frac{85}{100}(45)+\frac{85\times2\text{x}}{100}=45$
$\text{45}\Big[1-\frac{85}{100}\Big]=\frac{85\times2\text{x}}{100}$
$\frac{45\times15}{100}=\frac{85\times2\text{x}}{100}$
$\text{x}=\frac{45\times15}{2\times85}=\frac{135}{34} \ \text{per} \ \text{kg}$$=3.97 \ \text{per} \ \text{kg}$
Reduced price$=\frac{85}{100}\times3.97=\frac{337.45}{100}$$=3.3748=3.38 \ \text{per} \ \text{kg}$

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