Question

One day, Ted needed a pen. When he reached the store, he found that the shopkeeper had lost his list on which he had written how many notebooks he sold. But he had the list of pens and his total profit.

For each notebook, he gets $Rs4$ as profit and for each pen he loses $20$ paisa. The total profit he made was $Rs40$. And he sold $100$ pens during that period. Find the number of notebooks sold during that period. [Assume he only sold notebooks and pens during that period; Answer in terms of a number]

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Answer 1

Loss on selling $1$ pen = $20$ paisa = $-Rs0.2$

So, loss on selling $100$ pens $=Rs\left(\right(-0.2)\times 100)=$ $-Rs20$

Total gain = Profit earned on notebooks + Loss incurred on pens

So, Profit earned on notebooks = Total gain - Loss incurred on pens

$=Rs40-(-Rs20)$

$=Rs40+Rs20$

$=Rs60$

So, notebooks sold = Profit earned on notebooks/ Profit per notebook

$=Rs60÷Rs4$

$=15notebooks$