4. (20 points) Lisa can get a basic model of iPhone for $600 without having to sign up with a carrier. If she signs a contract with Verizon, she can get it for $200. To make up for the difference, Verizon requires Lisa to make constant monthly payments for 2 years (total of 24 payments) starting from a month from the purchase date. At the time of the purchase, annual interest rate is 6%. After a year, Verizon increases the constant monthly payment for the rest of the contract due to an increase in the interest rates from 6% to 9%. By how much should the constant monthly payment be increased so that Verizon exactly makes up for the discounted iPhone price