‘Mate in 3? Ok!’ is the title of this chess app. The program has 210 problems. There are 10 free levels and I’ve done them all. The next 200 levels cost $1.99. Which comes to 1 penny per puzzle.
I know lots of people are reluctant to make such a purchase but are perfectly happy to spend $4.50 on a cup of coffee that lasts 15 minutes.