Michael holds up nine fingers and starts counting down as he puts down one finger at a time. He stops when he is holding up four fingers. Nine, eight, seven, six, five, four. Decide whether he was adding or subtracting by picking the number sentence that describes what he did. Nine plus four equals 13, nine take away five equals four, or five plus four equals nine.
We know that Michael was counting down. He started at number nine and put down one finger at a time, nine, eight, seven, six, five, four. Michael was subtracting. He was taking away or putting down one finger each time. Nine take away one is eight. Take away another one is seven. Seven take away one is six. Six take away one is five. And five take away one is four. So Michael started with nine fingers and he put down a total of five. Which of our number sentences explains this?
Both of these number sentences start with nine or nine fingers. The first sentence has a plus sign. Did Michael add more fingers? No, he put fingers down. He took fingers away. Let’s see if the second number sentence is true. Michael started with nine fingers. He put five down. And he ended with four fingers. That’s exactly what Michael did. If Michael was holding up nine fingers and started counting down as he put one finger down at a time and stopped when he reached four fingers, he was subtracting.
The number sentence that describes what he did is nine take away five equals four.