Answer: It's not reasonable.
Explanation:
Hi, to answer this question we have to add both book’s prices: 
15.25 + 4.85 = $20.1 
If he wants to pay with one $20 bill, it wouldn’t be reasonable, because: 
20 <20.1 
For an useful estimate, estimate to the nearest tenth: 
15.30 +4.90 = 20.2 
Still, he can’t afford the books because he is lacking 20 cents. 
Feel free to ask for more if needed or if you did not understand something.