Use the top-level ratio 4:5 to find how many treadmills and bicycles there are, then break each group down using sub-ratios.
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• Top-level ratio 4:5 means 9 units in total.
• 72 ÷ 9 = 8 per unit.
• Treadmills: 4 × 8 = 32
• Bicycles: 5 × 8 = 40
For the treadmills (sub-ratio 5 + 3):
• 8 parts = 4 per part
• occupied: 5 × 4 = 20
• vacant: 3 × 4 = 12
For the bicycles (sub-ratio 7 + 3):
• 10 parts = 4 per part
• occupied: 7 × 4 = 28
• vacant: 3 × 4 = 12
Total vacant: 12 + 12 = 24.
Check: 32 + 40 = 72 = total (72). ✓
Final Answer: 24