Question

Sophie Ruth is eating a 50-gram chocolate bar which contains 30%, percent cocoa.

Answer 1

Well , 10% is 5 g so if you times those 5 g by 3 it is 15 g there are 15 grams of cocoa in the chocolate bar <span />

Answer 2

 the mass of the chocolate bar is 50 g. 
we are told that 30 % of the bar consists of cocoa.
percentages are values given by a fraction of 100.
therefore for every 100 g of the chocolate bar, 30 g of the bar is cocoa.
so if 100 g of bar contains - 30 g cocoa 
then 50 g of the bar contains - $\frac{30}{100} * 50 = 15$
therefore 15 g of the chocolate bar is cocoa