列方程解题：1.三年级有188人,比四年级人数的3倍多41人.

     设四年级有X人.
          3X+41=188
                3X=147
                            X=49
             答：四年级有49人.
 
2.           设五年级有X人.
                 2X-39=49
                      2X=88      
                         X=44
              答：五年级有44人.
 
3.            设水星绕太阳一周大约要X天
                      4X+13=365
                           4X= 352
                               X=88
                      答：水星绕太阳一周大约要88天
 
4.               设B大楼的高度是X米
                        7X-54=492
                               7X=546
                                X=78      
                答：B大楼的高度是78米