In the sum of the expression , what digit will be in the tens place?
Source: NCTM Mathematics Teacher, February 2008
We are adding nineteen numbers from the smallest to the largest . Note that there are nineteen in the ones place and eighteen in the tens place. First, we add the nineteen in the ones place and get . Consider the number . The in the ones place of will be the ones place digit of the final sum. The in the tens place of means and if we regroup with the eighteen , we get . The digit in the tens place of the final sum is .