Use the top-level ratio 5:4 to find how many adult and child there are, then break each group down using sub-ratios.
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• Top-level ratio 5:4 means 9 units in total.
• 180 ÷ 9 = 20 per unit.
• Adult: 5 × 20 = 100
• Child: 4 × 20 = 80
For the adult (sub-ratio 1 + 4):
• 5 parts = 20 per part
• take lessons: 1 × 20 = 20
• don't: 4 × 20 = 80
For the child (sub-ratio 3 + 1):
• 4 parts = 20 per part
• take lessons: 3 × 20 = 60
• don't: 1 × 20 = 20
Child take lessons: 60. Adult take lessons: 20.
Difference: 60 - 20 = 40.
Check: 100 + 80 = 180 = total (180). ✓
Final Answer: 40