x - liczba atlasów
10%x = 1/10 x
[tex]\frac{1}{10}x - 5 = \frac{1}{30}x\\\\\frac{1}{10}x - \frac{1}{30}x = 5\\\\\frac{3}{30}x - \frac{1}{30}x = 5\\\\\frac{2}{30}x = 5\\\\\frac{1}{15}x = 5 \ \ \ |\cdot15\\\\\boxed{x = 75}[/tex]
Odp. W bibliotece było 75 atlasów.
