457) 15 + (90 - a) : 33 = 17

Решение:

1. \( 15 + (90 - a) : 33 = 17 \)
2. \( (90 - a) : 33 = 17 - 15 \)
3. \( (90 - a) : 33 = 2 \)
4. \( 90 - a = 2 \cdot 33 \)
5. \( 90 - a = 66 \)
6. \( a = 90 - 66 \)
7. \( a = 24 \)

Ответ: a = 24.