Each month the balance grows by the annual percentage rate divided by 12, then the payment is subtracted. The page counts months until the balance is gone and adds the interest along the way. If the payment does not cover that month’s interest, the balance is not paid off.
A $5,000 balance at 20% APR grows by about $83 in the first month before the payment. A $150 payment covers that interest and reduces the balance. A payment below the interest would not. The months to zero depend on the payment you type.
Fees, a promotional rate that expires, and a changing minimum payment are not included. The rate stays at the APR you enter for the whole payoff.