The pharmacy has an old scale which has only two measuring weights 30 grams and 5 grams. How can you divide 300 grams of powder medicine into 3 groups, one 150 grams, one 100 and the other 50 grams. You can only weigh 3 times.
1. Keep 30 gms in one pan (pan A) of the weighing scale/balance and keep 5 gms in another (pan B). Weigh powder in pan B. So effectively you have 25 gms powder now. 2. Keep 25 gms powder & 30 gms in pan A and 5 gms in Pan B. Weigh powder in pan B. You have effectively 50 gms powder in pan B now. 3. Keep 25 gms & 50 gms powder along with 30 gms weight in pan A and 5 gms weight in pan B. Measure 100 gms powder in Pan B now.
This is your 100 gms pack at step 3. Step 2 gave you 50 gms pack. And what is left is 150 gms pwder out of 300 gms total.