Use the top-level ratio 4:3 to find how many junior and senior there are, then break each group down using sub-ratios.
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• Top-level ratio 4:3 means 7 units in total.
• 84 ÷ 7 = 12 per unit.
• Junior: 4 × 12 = 48
• Senior: 3 × 12 = 36
For the junior (sub-ratio 1 + 3):
• 4 parts = 12 per part
• compete: 1 × 12 = 12
• don't: 3 × 12 = 36
For the senior (sub-ratio 2 + 1):
• 3 parts = 12 per part
• compete: 2 × 12 = 24
• don't: 1 × 12 = 12
Total compete: 36. Total don't compete: 48.
Difference: |36 - 48| = 12.
Check: 48 + 36 = 84 = total (84). ✓
Final Answer: 12