Library

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

Correct result:

o =  825

Solution:

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

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