Depends how big the grain of salt is.
Buy some scales and find out
4 tbsp of salt is approximately 60 grams
One cubic centimeter (cc) of salt typically weighs about 2.16 grams.
It weighs approximately 6 grams.
5 grams 0.5 Newtons 0.5 centigrams 0.005 kilograms
100ml of water weighs approximately 100 grams. Adding 15 grams of salt would bring the total weight to 115 grams.
You have said yourself that is about 200 grams.
Weigh it in grams, then, as the density of table salt is about 2.17 g/cc, divide the weight by 2.17 and the answer will be the volume of salt in cc.
There will be a bit of variation in a measurement because salt crystals are not a uniform size. Now the good news, thanks to the Salt Institute, which is a non-profit trade association for the salt industry. They have the straight scoop (no pun intended) on salt. Salt weighs about 6 grams per teaspoon or about 18 grams per tablespoon. That makes your 3 tablespoons of salt weigh in at about 54 grams.
You can not equate volume and weight without knowing the density of the substance being measured.A tablespoon of feathers would not weigh as much as a tablespoon of salt.
Weight and length are not comparable without other factors. I assume you meant milligrams though. There are 2900 milligrams in 2.9 grams of salt.