7) (c : 9) * 15 - 47 = 28

Решение:

1. \[ (c : 9) \times 15 - 47 = 28 \]
2. \[ (c : 9) \times 15 = 28 + 47 \]
3. \[ (c : 9) \times 15 = 75 \]
4. \[ c : 9 = 75 : 15 \]
5. \[ c : 9 = 5 \]
6. \[ c = 5 \times 9 \]
7. \[ c = 45 \]

Ответ: c = 45