By selling a pen for Rs. $36$, a shopkeeper gains $20\%$. The cost price is

By selling a pen for Rs. $36$, a shopkeeper gains $20\%$. The cost price is
  1. Rs. $28$
  2. Rs. $30$
  3. Rs. $32$
  4. Rs. $34$

Solution

$\text{CP} = \dfrac{\text{SP} \times 100}{100 + g} = \dfrac{36 \times 100}{120} = 30$.

Asked in: IMO

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