The total number of binders the planner put together for attendees and presenters is 280 + 31 = 311. Adding the ones place (0 + 1) results in the digit 1, adding the tens place (8 + 3) results in the number 11, which should be recorded as a 1 in the tens place and a 1 carried to the hundreds place, then adding the hundreds place (2 + 1) results in the digit 3.
211 results from not carrying a 1 to the hundreds place after adding the tens place. 249 is the result of subtracting the presenter binders from the attendee binders. However, the total number of binders will be found through addition, not subtraction. 590 is incorrect. This answer results from incorrectly adding the numbers.