Weighted average: multiply each group's average by its ratio weight, then divide by total weight.
• Group 1: 70 × 2 = 140
• Group 2: 80 × 3 = 240
• Group 3: 90 × 5 = 450
• Sum = 140 + 240 + 450 = 830
• Total weight = 2 + 3 + 5 = 10
• Combined average = 830 ÷ 10 = 83.
Final Answer: 83