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A shopkeeper buys a number of books for ₹1,800. If he had bought $15$ more books for the same amount, then each book would have cost him ₹20 less. Find how many books he bought initially.
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Let the number of books bought initially be $x$
According to question,
$\frac{1800}{x} - \frac{1800}{x+15} = 20$
$x^2 + 15x - 1350 = 0$
$(x + 45)(x - 30) = 0$
$x = -45$
$\therefore x = 30$
So, the number of books bought initially $= 30$
According to question,
$\frac{1800}{x} - \frac{1800}{x+15} = 20$
$x^2 + 15x - 1350 = 0$
$(x + 45)(x - 30) = 0$
$x = -45$
$\therefore x = 30$
So, the number of books bought initially $= 30$