All you are doing wrong is dividing by 25 when it should be 0.25
you've correctly got 62.5 microg to 0.0625 millig
Now top and bottom are both in mg so 0.0625 divided by 0.25 = 0.25
i think the answer is misleading as it clearly doesnt show the decimal point before 25.........however im still struggling to get my head round it so i will have another read through your replies and a massive thankyou too !!!