A loan of $50,000 is to be repaid with a series of 15 annual payments beginning two years after the date of the loan. The first five payments (X each) are half as much as the second five payments which are half as much as the final five payments. Interest is calculated at an effective annual rate of 7% for the first six years, and at an effective annual rate of 4% for the last ten years of the loan. Determine X.