3.1. Every day, a sweet tooth buys one more candy than the previous day. In one week, on Monday, Tuesday, and Wednesday, he bought a total of 504 candies. How many candies did he buy on Thursday, Friday, and Saturday in total for the same week?
Answer: 513

Solution. On Thursday, 3 more candies were bought than on Monday, on Friday - 3 more than on Tuesday, and on Saturday - 3 more than on Wednesday. In total, 9 more candies were bought on Thursday, Friday, and Saturday compared to Monday, Tuesday, and Wednesday, i.e., $504+9=513$ candies.