Two decreased by a half of a number is 1/2 of the number

 The number is:  "2" .

The expression is:  2 − (½)x = (½)x  ;
                → in which "x" is the "unknown number".

Explanation:

Given:  

"Two decreased by a half of a number is one-half of the number" ; 

Let "x" represent the "unknown number" for which we wish to solve;

Write the expression as:

" 2 − (½)x = (½)x " .

Note:  Add "(½)x" to EACH SIDE of the equation:

2 − (½)x + (½)x = (½)x + (½)x ;

to get:

       →  2 = 1x ; 

       ↔  1x = 2 ;
 
       ↔ x = 2 .

 The number is:  " 2 " .