Use the top-level ratio 11:4 to find how many amateurs and professionals there are, then break each group down using sub-ratios.
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• Top-level ratio 11:4 means 15 units in total.
• 75 ÷ 15 = 5 per unit.
• Amateurs: 11 × 5 = 55
• Professionals: 4 × 5 = 20
For the amateurs (sub-ratio 9 + 2):
• 11 parts = 5 per part
• use digital: 9 × 5 = 45
• film cameras: 2 × 5 = 10
For the professionals (sub-ratio 3 + 1):
• 4 parts = 5 per part
• use digital: 3 × 5 = 15
• film cameras: 1 × 5 = 5
Total film cameras: 10 + 5 = 15.
Check: 55 + 20 = 75 = total (75). ✓
Final Answer: 15