How to determine how much principal remains when balloon payment is due

[Aidan Birmingham]

I am closing a seller financing deal but am confused with how monthly payments are split up between principal and interest. How do I determine how much principal is owed at the 10 year mark for the balloon payment? I have looked at amortization tables but the payments seem to be almost 75% interest in the first 10 years. Is there a good way to structure the agreement? 

The terms are below: 

Sale Price: $230,000

- Down Payment: $5,000

- Loan Amount: $225,000

- Interest Rate: 3.75%

- Loan Term: 40 years (480 months)

- Interest-only for the first 1.5 years

-Year 10 ballon payment due: X (Refi Loan with bank)