To calculate IWAA:
1. Multiply each borrower's income by their age.
2. Add results.
3. Divide by the total income.
Example:
- John: $10,000 × 50 = $500,000
- Janet: $5,000 × 22 = $110,000
- Total: $610,000
- Total income: $15,000
IWAA = $610,000 ÷ $15,000 = 40.67
IWAA is 40.67 years.