This calculates the payments on the first and second mortgage that equal 90% and the blended rate which is compared to a 90% loan requiring PMI. Most loans greater than 80% loan-to-value requires Private Mortgage Insurance.
A combination of an 80% first loan and a 10% second loan allows the buyer to only have 10% cash investment and avoid the expense of PMI. The second loan could come from a conventional lending source or possibly from the seller. It must be disclosed in the sales contract