13分之2*11分之3*7分之4+13分之3*11分之2*7分之3
=2/13*3/11*4/7+3/13*2/11*3/7
=2/13*3/11*4/7+2/13*3/11*3/7
=2/13*3/11*(4/7+3/7)
=6/143