not a math guru but work in the mortgage field and have a question I am hoping someone can answer.

Scenario: one loan for $ 200,000 at 5pct over 30 years has a monthly principle and interest payment of $ 1074.

If broken up into two loans, each for $ 100,000, one loan with a 4% interest rate and the other with a 6pct interest rate, the payments are $ 477 and $ 600 respectively. Totaling $ 1077.

Considering that the combination scenario has an average rate equal to 5% and the amortization schedules each end at 360 months…… why the difference in total payments?

This payment difference increases with larger loan amounts and bigger swings in interest rates (e.g. 2% and 8%) for the same average rate.

Thanks in advance!