Apples investment

The merchant bought a certain amount of apples and paid one denarius for every seven apples. The next day he sold all the apples for one denarius for five pieces and earned 12 denarii. How many apples did he originally buy, and how many did he have to invest in this business?

Correct answer:

x =  210
y =  30

Step-by-step explanation:

$$\begin{gathered} 1/5\cdot x-1/7\cdot x=12 \\ 2x=420 \\ x=\frac{420}{2}=210 \\ x=210 \end{gathered}$$

$y=1/7\cdot x=1/7\cdot 210=30$

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