1/1X3=1/2(1/1-1/3)
1/2*4=1/2(1/2-1/4)
.
1/1X3+1/2X4+1/3X5+.+1/18X20
=1/2(1/1-1/3+1/2-1/4+1/3-1/5+.+1/18-1/20)
=1/2(1/1+1/2-1/19-1/20)
=531/760