A person sold a book for Rs 21 and got a loss percentage which was numerically equal to the cost price. What is the cost price (in Rs) of the book?
Selling price of book = Rs. 21
Let cost price = Rs. $$x$$
=> Loss % = $$x\%$$
According to ques,
=> $$\frac{(x-21)}{x}\times100=x$$
=> $$100x-2100=x^2$$
=> $$x^2-100x+2100=0$$
=> $$x^2-30x-70x+2100=0$$
=> $$x(x-30)-70(x-30)=0$$
=> $$(x-30)(x-70)=0$$
=> $$x=30,70$$
$$\therefore$$ Cost price (in Rs) of the book = Rs. 30 or 70
=> Ans - (C)
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