解:(1)12÷1.2-18×

,
=10-6,
=4;
答:差是4.
(2)设这个数为x,得
x+2.1÷

=5.4,
x+2.1×

=5.4,
x+2.4=5.4,
x=3;
答:这个数是3.
分析:(1)求差,就要知道被减数和减数分别是多少,由题意,被减数是“12除以1.2的商”,即12÷1.2;减数是“18的

”,即18×

,由此列式为12÷1.2-18×

,计算即可;
(2)要求用方程解答,可设这个数为x,2.1除以

的商是2.1÷

,与x的和是5.4,由此列方程为x+2.1÷

=5.4,解此方程即可.
点评:(1)从问题出发,看看要求的是什么,从题中找出解决问题所需的条件,列式解答;
(2)此题考查了学生列方程解方程的能力.在解方程时,依据等式的性质.