Five deposits of 650 are made into a fund at 3-year intervals with the first deposit at the beginning of the first year. The fund earns interest at an annual effective rate of 3% during the first six years and at an annual effective rate of 6% after that. There are additional deposits of 600 at t=21 and 400 at t=24. Find the 3-year interest rate that would prevail during the first 6 years