So this is what we know:
There are only 0.5 kilograms of medicine in the bottle
1 child dose = 20 grams
1 adult dose = 45 grams
We need to give 12 adults and 10 children the medicine, so c = 10 and a = 12
Inserting that those values into the formula, you get a total of 740 grams
740 grams = 0.74 kilograms
Because 0.74 > 0.5 grams in the bottle, there is not enough medicine in the bottle.
To find out how much more you will need, subtract 0.5 from 0.74 to get 0.24.
You need an additional 0.24 kilograms of the medicine to help all of the people.