You need 1.3 g of calcium per day. You get 290 mg of calcium per glass of milk. If you drink 4 glasses of milk, how much more calcium do you need from other sources?
Since there are 1000 mg in a gram, you need 1300 mg of calcium. You then multiply 290 by 4 which is 1160 mg. Then, to find the remainder, you subtract 1160 from 1300 which is 140 mg from other sources.
