Sean sold 342 boxes of cupcakes last week and 557 boxes this week. To find the increase in sales, we subtract last week's amount from this week's, giving 557 - 342 = 215. Another method is to write the numbers as 4986 - 2354 = 2632 to find the difference between them by arranging the digits in columns and subtracting. Subtraction allows regrouping of digits when the number in the top row is less than the number below it in a column.