lesson
Updated 1 month ago · 6 views
Imagine you have 7 shiny marbles in your pocket, and you give 2 of them to your best friend. How can you figure out how many marbles are left without counting all the way up from zero?
We can use a quick mental trick called counting back! When you count back, you start at your whole number and hop backward like a frog in reverse.
The Number Line
A number line puts numbers in order from 0 to 10. When we add, we hop right toward bigger numbers, but when we subtract, we hop left toward smaller numbers.
📊Interactive diagram
What happens when we take away just 1 from a number?
Subtracting 1
To solve 6−1, place your finger on 6 and take one hop back to the left. You land on 5, so 6−1=5.
📊Interactive diagram
Counting back 1 is easy because it is always just the number that comes right before!
What if you need to take away 2 or 3?
Subtracting 2 and 3
To solve 8−3, start at 8 and make three hops back while counting: "7... 6... 5!"