2 Answers

16 is to 20 as x is to 100.  
16/20 = x/100    
 If each book cost $5 to buy, 20 books would cost $100.  If you sold 16 books for the cost of 20, you would sell 16 books for $100.  16 books would have cost YOU $80 (16 x 5).  That gives you a profit of  $20.   
20/100 = .20 = 20%  
If each book cost $4 to buy, 20 books would cost you $80.  You would sell 16 for $80 (which would have cost you $64 (16 x 4).  Your profit would be 80 - 64 = $16.  16/80 = .20 = 20%
Your profit is 20 % 
Play with it like that... You'll always come up with 20% profit. 

Surely the point is that we are buying 20 books for the same price we are selling 16 of that at.

If you buy the books at £1.00 each = £20, and If we sell 16 of the books to achieve an income of £20 them we sell them at £1.25. The answer is nil.   The questioner does not ask about any potential profit from the 4 unsold books which would give him a profit of 25% if he sold them.