(56/4+5)*3-3=54,倒推法
反过来,56除以4,加5,乘以3,减3,为54.
54你从后往前算，就推出来啦56/4+5*3-3=54
倒着算呗56除以4,=14,再+5,=19,然后乘3,=57,最后-3,=54
54
[(X+3)/3-5]*4=56,得X=54
