Which of these people repaid his loan at a variable interest rate?

The people who repaid his loan at a variable interest rate was : Gary who paid $ 37 each month for the first 6 months and $ 67 for the next 6 months.
When you pay with variable interest rate, the total interest will change from time to time. As we see, there are a difference of payment from the first 6 months compared to the next 6 months, indicating a different interest rate.