math
posted by sarah .
Marissa plans to use 50 sheets of paper for a scrapbook. how much thicker will the scrapbook be if she uses heavyweight instead or lightweight paper?
100 sheets of lightweight in thickness is 3/10 in
100 sheets of heavyweight in thickness is 2/5 in.

2/5 = 4/10
[(4/10)  (3/10)] / 2 = ____ inch 
is the answer 1/20 if not I am sooo confused.