4/5*7+4/7*9+4/9*11+.+4/53*55
=2*(2/5*7+2/7*9+2/9*11+...+2/53*55)
=2*(1/5-1/7+1/7-1/9+...+1/53-1/55)
=2*(1/5-1/55)
=2*10/55
=4/11