David takes 300 milligrams of medicine every day. How many grams is this?

The metric system works in quantities of 10. As a quick example, there are 10 millimetres in each centimetre.

David is taking 300 milligrams of medicine. Let's follow the quick example above and figure out how many centigrams that is. There are 10 milligrams to every centigram, so we can say that David is taking 30 centigrams of medicine. (See that? Because there are 10 milligrams in every centigram, we can divide the 300 by 10 and get 30 centigrams).

The next biggest measure is decigrams. There are 10 centigrams to every decigram. So we can divide by 10 and see that David is taking 3 decigrams of medicine (again - this is the same amount of medicine each time - we're just using a bigger and bigger measure each time).

The next bigger measure if grams. There are 10 decigrams to every gram, so we can once again divide by 10 and arrive at 0.3 grams of medicine.

Let's summarize:

300 milligrams of medicine is the same as 30 centigrams of medicine is the same as 3 decigrams of medicine is the same as 0.3 grams of medicine.

Each time we use a bigger measure (whether is a stick for measuring length, or a scale for weight, or a bottle for liquid volume, or whatever it is), we divide the measured amount by 10. If we go the other way and use a smaller measure, we multiply by 10.

To convert milligrams to grams, divide the number of milligrams by 1000.

[ 300 \text{ milligrams} = \frac{300}{1000} \text{ grams} = 0.3 \text{ grams} ]

So, 300 milligrams of medicine is equal to 0.3 grams.