2.4x+3.4=45.2, 2.4x=45.2-3.4=41.8, x=41.8/2.4=418/24=209/12=175/12.
This gives a unique value for x.
An inequality doesn't produce unique values but rather a range of values, so if we convert an equality into ask inequality, we are removing the unique solution and are introducing a range of values (usually an infinite range). This is rather like having a clear, high-definition photo, and then deliberately blurring the image so as to produce a lower grade photo.
In this case you could write the inequality 2.4x<45.2 so x<45.2/2.4, x<18⅚.
It's true that 175/12<18⅚, but then so many other numbers are also less than 18⅚ but only one gives the unique solution to the given equation.