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On selling a pen at $5\%$ loss and a book at $15\%$ gain, Karim gains Rs. $7$. If he sells the pen at $5\%$ gain and the book at $10\%$ gain, he gains Rs. $13$. What is the cost price of the book in Rupees?

  1. $80$
  2. $85$
  3. $95$
  4. $100$
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Let $100x$ be the cost price of the pen, and $100y$ be the cost price of the book.

  • On selling a pen at $5\%$ loss.
  • On Selling a book at $15\%$ gain.

He gains $7$ rupees. So, equation will be :

$\require{cancel} \begin{array} { c c } \text{Pen}  &  \text{ Book} \\  \cancel{100}x\times \frac{-5}{\cancel{100}} &  \quad \cancel{100}y\times \frac{15}{\cancel{100}} \end{array}$

$\boxed{-5x+15y = \text{Rs.}\:7} \quad \longrightarrow (1)$

  • On selling a pen at $5\%$ gain.
  • On selling a book at $10\%$ gain.

He gains $13$ rupees. So, equation will be :

$\require{cancel} \begin{array} { c c } \text{Pen}  & \text{ Book} \\  \cancel{100}x\times \frac{5}{\cancel{100}} &   \quad\cancel{100}y\times \frac{10}{\cancel{100}} \end{array}$

$\boxed{5x+10y = \text{Rs.}\:13}\quad \longrightarrow (2)$

From equation $(1),$ and $(2),$ we get

$\begin{matrix} {\cancel{-5x}} +15y =7 \quad \longrightarrow (1) \\ \underline{{\cancel{-5x}} +10y =13} \quad \longrightarrow (2)\\ 25y=20 \end{matrix}\\ \Rightarrow\boxed{y=\frac{20}{25} = \frac{4}{5}}$

Now, the cost price of book $ = 100y = 100\times\frac{4}{5} = 80$

$\therefore$ The cost price of book is $\text{Rs.}\; 80.$

Correct Answer $: \text{A}$

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