Suthie ruth is eating 50-gram chocolate bar which contains 30% cocoa. how many grams of cocoa are in the chocolate bar

15 grams of coca in a 50 gram chocolate bar.

Well, this question can be restated as suthie ruth is eating 50 gram chocolate bar which contains 30% OF cocoa. 
   Anyway, in math, OF means multiplication. And as you know, % is over 100. So you can simply say;
50 x 30/100. And that will be 15. So the answer is 15grams. Hope i helped. Have a nice day