Betty had 250 books and magazines. 30% of them were fiction books, 60% of the remainder were magazines and the rest were non-fiction books. Paul had no non-fiction books at first. Then Betty gave Paul some non-fiction books and Paul had 2/3 as many non-fiction books as Betty in the end. What percentage of the total number of her books and magazines did Betty give to Paul?
Betty's books:
fiction --- 250(.3) = 75
mags ---- .6(250-75) = 105
non-fiction ----- 250-75-105 = 70
after book exchange
Paul nonfiction ---- x
Betty non-fiction --- 70-x
x = (2/3)(70-x)
3x = 140-2x
5x = 140
x = 28
You do the last step