Library

We purchased new books for the library. Five-eighths were professional books, one-fifth were encyclopedias, and 231 books were dictionaries. How many professional books were there?

Correct answer:

o =  825

Step-by-step explanation:

$$\begin{gathered} \frac{5}{8}n+\frac{n}{5}+231=n \\ 5/8\cdot n+n/5+231=n \\ 7n=9240 \\ n=\frac{9240}{7}=1320 \\ n=1320 \\ o=\frac{5}{8}\cdot n=\frac{5}{8}\cdot 1320=\frac{5\cdot 1320}{8}=\frac{6600}{8}=825 \end{gathered}$$

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