21.

Решение:

1. \( 250 + 250 \cdot 2 = 250 + 500 = 750 \)
2. \( (250 + 250) \cdot 2 = 500 \cdot 2 = 1000 \)
3. \( (480 + 120) : 3 = 600 : 3 = 200 \)
4. \( 60 + 90 : 10 \cdot 3 = 60 + 9 \cdot 3 = 60 + 27 = 87 \)
5. \( 60 + 90 : (10 \cdot 3) = 60 + 90 : 30 = 60 + 3 = 63 \)
6. \( 120 - 80 : 4 \cdot 5 = 120 - 20 \cdot 5 = 120 - 100 = 20 \)
7. \( 84 - 24 + 48 : 6 = 84 - 24 + 8 = 60 + 8 = 68 \)
8. \( 84 - (24 + 48) : 6 = 84 - 72 : 6 = 84 - 12 = 72 \)
9. \( 100 - 64 + 36 : 4 = 100 - 64 + 9 = 36 + 9 = 45 \)

Ответ: 750, 1000, 200, 87, 63, 20, 68, 72, 45.