你定义的新运算是 a*b = a*b/(a+b)的意思吗?如果是,计算如下：
2*10= 2*10/(2+10) = 20/12=5/3
5/3 * 10 = 5/3 * 10 /(5/3 + 10) = 5*10*3/(5+10*3) = 150/35 = 30/7