please give the answer and steps:

4. (6 points) In 2020, Daniel will receive annuity income at the end of each month. The first payment of $1,000 is received at the end of January 2020 and each successive income is $50 more than the preceding month's income. Daniel has a savings account that credits interest at 1.80% compounded monthly, but he is also thinking to invest some of his annuity income into a fixed-income investment fund that earns at 7% annual effective rate. Daniel decides to invest a fraction r of his monthly annuity income to the investment fund and deposit the rest into his savings account, at the end of each month right after he receives each payment. The accumulated value of two accounts combined at the end of 2020 (including the December 2020 annuity income) is $15,500. Calculate r