40-9-21-7-(10-3-2-4)

Решение:

\( 40 - 9 - 21 - 7 - (10 - 3 - 2 - 4) \)

1. \( 10 - 3 = 7 \)
2. \( 7 - 2 = 5 \)
3. \( 5 - 4 = 1 \)
4. \( 40 - 9 = 31 \)
5. \( 31 - 21 = 10 \)
6. \( 10 - 7 = 3 \)
7. \( 3 - 1 = 2 \)

Ответ: 2