Subtraction within 20 by Counting
In this video, we will learn how to
count back to find the difference between two numbers within 20. We can count back on a number line
to help us take away. To take away on a number line, we
start with the first number. If we’re doing 15 take away five,
we start at number 15. And we have to take away five. So we need to count back five
places on the number line: one, two, three, four, five. 15 take away five is 10. We started at number 15. We counted back or took away
five. We finished on number 10. So if we want to subtract, we can
Subtracting or taking away is the
same as counting back in our heads or on a number line. To subtract, we count back. Let’s practice with an example.
Isabella put her counter on 12 then
moved it back two spaces. 10 is two less than 12. Help her find the missing
number. What is three less than 16?
We’re looking for the number that
is three less than 16. So Isabella needs to put her
counter on number 16. We need to find the number which is
three less than 16. So we need to count back three
places: one, two, three. The counter landed on number
13. 16 take away three is 13. Or we could say that 13 is three
less than 16. We started at number 16 on the
number line and counted back three spaces, because counting back is the same as
In this video, we’ve learned how to
count back to find the difference between two numbers within 20.