You can browse exercise keys on Euroki or GDZ.ru.

Units (Einheit) 1–7, including "Fitnesscenter Deutsch" and "Kleine Pause" sections.

Онлайн ГДЗ. Ответы к "Горизонты" рабочая тетрадь 5 класс

Solutions for the green-covered workbook often paired with the standard textbook.

This is the most popular series for German as a second foreign language.