Given the following cash flows and an interest rate of 5%:

t	Rt	vtRt	tvtRt
1	500	476	476
2	600	544	1,088
3	700	605	1,815
4	800	658	2,632
5	900	705	3,525
6	700	522	3,132
7	500	355	2,485
8	300	203	1,624
9	200	129	1,161
10	100	61	610
Sum	5,300	4,258	18,548

Find the modified duration.