4. $15,000 is deposited into a fund on January 1, 2017. Another deposit is made into the fund on July 1, 2017. Moreover, on January 1, 2018, the balance of the fund is $18,000. The time-weighted yield rate is 12% and the dollar-weighted yield rate is 11.5%. Calculate the yield rate of the fund during the first six months of 2017, quoted in terms of the annual effective interest rate