lesson
Updated 1 month ago · 7 views
3... 2... 1... Blastoff! Whenever you hear a rocket launch, you are listening to counting back in action.
We can use that same countdown trick to subtract small numbers like 1, 2, or 3 fast without drawing pictures.
Let's see what happens when you take away just one number.
Counting Back 1
To solve 7−1, start at 7 and take one hop backward on the number line. The number right before 7 is 6, so 7−1=6.
📊Interactive diagram
What if you need to subtract a slightly bigger number, like 2?
Counting Back 2
To solve 8−2, put 8 in your head and put up 2 fingers. Count back two numbers as you tap your fingers: 7, 6.
📊Interactive diagram
Can we use this same superpower to take away 3?
Counting Back 3
To solve 9−3, start at 9 and take three hops backward: 8, 7, 6. You land on 6, which means 9−3=6.