Write the missing digit to complete the standard algorithm.
Before we can fill in this missing digit, we’ll need to solve this problem: 721 times 13. Starting all the way on the right, we multiply three times one which equals three. Moving to the tens place, we multiply three times two equals six. Now, the hundreds’ place, three times seven equals 21. We’ll now use the one to multiply and we’ll go back to the units place and start again: one times one is one, one times two is two, one times seven is seven.
This completes our multiplication stage of the standard algorithm. And we start to add the two products we found. Starting with the units placed, three plus zero equals three. In the tens’ place, six plus one equals seven. In the hundreds’ place, one plus two equals three. And in the thousands’ place, two plus seven equals nine.
Using this solution, we can solve for the missing digit. The missing digit comes from the hundreds’ place. In the hundreds’ place, there should be a three.