Here are two number sentences which have a missing number. Nine minus six equals something. Six plus something equals nine. Which number completes both of the number sentences?
To help us solve this problem, we have nine little critters. Let’s use the critters to demonstrate nine minus six. Currently, we have nine critters. If six of them disappear, how many will be remaining? One, two, three, four, five, six. This shows us that nine minus six equals three.
For our second number sentence, we’ll need to start with six critters and add some number so that we end up with nine. Starting with six, we add one to make seven. If we add a second critter, we now have eight total. By adding one more, we’ve added three to our six original and have a total of nine. This illustrates that six plus three equals nine. The number that completes both of the number sentences is three.