If you pay $15,000 per year, then it will work as follows:
On the day you take out the loan, you owe $100,000 in principle and $0 in interest.
One year later, you owe $110,000 and pay $15,000, which leaves you owing $95,000, meaning you paid $10,000 toward the interest and $5,000 toward the principle.
One year later, you owe $104,500 (=$95,000+$9,500), and pay $15,000, meaning you paid $9,500 toward the interest and $5,500 toward the principle.
One year later, you owe $98,450 (=$89,500+$8,950), and pay $15,000, meaning you paid $8,950 toward the interest and $6,050 toward the principle.
And so on.
You still could pay off the loan with payments that follow a rule of half going to interest and half going to principle, but that would require the first year's payment to be $20,000, and then the next year's to be $18,000, and then $16,400, and so on. It wouldn't be the same payment every time.