3/10+3/40+3/88+3/154+3/238+3/340
=3/2*5+3/5*8+3/8*11+3/11*14+3/14*17+3/17*20
=1/2-1/5+1/5-1/8+...+1/17-1/20
=1/2-1/20
=9/20