13/49×21+13×28/49=13/49 ×(21+28)=13/49 ×49=13
13/49*21+13*28/49=13*(21/49+28/49)=13
13/49×21+13×28/49=13/49*(21+28)=13/49*49=13
13/49×21+13×28/49=13/7*3+13*4/7=13/7*(3+4)=13