Answer: 15 grams
If he had 100 grams of candy bar, then 30% of that is 30 grams (since 30/100 = 30%). Cut this in half and we end up with 30/2 = 15.
Another way to find the answer is to multiply 50 and 0.30 which is the decimal form of 30%. So we have 50*0.30 = 15 which is the same answer.