1.4. A ballpoint pen costs 10 rubles, a gel pen costs 30 rubles, and a fountain pen costs 60 rubles. What is the maximum number of ballpoint pens that can be bought if you need to buy exactly 20 pens in total and among them there must be pens of all three types, and you need to spend exactly 500 rubles on them?
Answer: 11. Comments. The solutions are two triples of integers $(11,5,4)$ and $(8,10,2)$.