A craftsman can sell 10 jewelry sets for $500 each. He knows that for each additional set he makes, the price of each set will decrease by $25. How many jewelry sets should he make if he wants to maximize his earnings?

The max warning happens when he makes 15 sets.

Here is what happens as he makes more and more sets:

10 sets: $500 each

Total earned: 10*500=5000

11 sets: $475 each

Total earned: 11*475=5225

12 sets: $450 each

Total earned: 12*450=5400

13 sets: $425 each

Total earned: 13*425=5525

14 sets: $400 each

Total earned: 14*400=5600

15 sets: $375 each

Total earned: 15*375=5625

16 sets: $350 each

Total earned: 16*350=5600

17 sets: $325 each

Total earned: 17*325=5525

18 sets: $300 each

Total earned: 18*300=5400

19 sets: $275 each

Total earned: 19*275=5225

20 sets: $250 each

Total earned: 20*250=5000

21 sets: $225 each

Total earned: 21*225=4725

22 sets: $200 each

Total earned: 22*200=4400

23 sets: $175 each

Total earned: 23*175=4025

24 sets: $150 each

Total earned: 24*150=3600

25 sets: $125 each

Total earned: 25*125=3125

26 sets: $100 each

Total earned: 26*100=2600