First let's make sure we identify the standard quantity of our units; we have grams that we need to convert to kilograms. Kilo means the same as 1,000 so there are 1000 grams in one kilogram.
Now, let's see how much all the items in the sandwich weigh:
15 grams cheese-plus
50 grams chicken-plus
3x20 gram slices of bread (so we have a total of 60 grams of bread0
added together we get 125 total grams of everything the sandwich is made of
now we need to convert that to kilograms, so we set this up the following way to get grams cancelled out and we end up with kilograms
125 grams X 1 kilogram/1000 grams = 125/1000 kilograms or .125 kilograms