1 ml of reconstituted solution has 500 mg in it.
In order to administer 250 mg of this medication, you would administer 1/2 mL as 250 is 1/2 of 500.
To solve this question using dimensional analysis, you solve by X and plug in the numbers provided in the example:
?x ml: 1 ml x 250 mg = 1 ml
500 mg dose ordered 2