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If Petra read four pages more a day, she would read the book two days earlier than if she read six pages a day. How many pages does the book have?

Correct answer:

n =  30

Step-by-step explanation:

$$\begin{gathered} n=6\cdot d \\ n=(6+4)\cdot (d-2) \\ 6d-n=0 \\ 10d-n=20 \\ Pivot:Row1\leftrightarrow Row2 \\ 10d-n=20 \\ 6d-n=0 \\ Row2-\frac{6}{10}\cdot Row1\to Row2 \\ 10d-n=20 \\ -0.4n=-12 \\ n=\frac{-12}{-0.4}=30 \\ d=\frac{20+n}{10}=\frac{20+30}{10}=5 \\ d=5 \\ n=30 \end{gathered}$$

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