i think, the time is weird, but this is my idea..

compute for the accumulated value of $50,000.00 on Aug.17,2008 and re-compute for the new payment until the next 9 years after subtracting the accumulated value from $100,000..

or you can also compute for the present value equal to $100,000 of the sum of $50,000 paid on 1/22/2008 and every R's paid on the 17,Aug. until the 10th year, and solve for R..

P.S. i really wanted to solve it myself, but it is too late here.. have to sleep.. Ü