Subtracting using a Number Line
First we will start with whole numbers.
30 – 12 = ?
Draw the number line. The smaller number (the one being
subtracted) goes on the left, and the bigger number (the one
you are subtracting from) goes on the right.
12
30
Next decide what round number (multiple of 10) is bigger and
12 and mark it on the number line, and what multiple of 10 is
less than 32. Mark that also.
12
20
30
Now since distances can be measured on a number line, find
the distance between each set of marked points and write
those answers above the line.
8
12
10
8 + 10 = 18
20
30
Add up those distances, to get your answer, which is 18. It
seems like extra work but this is how young children (second
graders) are being taught to subtract when they don’t know
how to regroup, or borrow. It turns a subtraction problem into
an addition problem.
Now let’s try it with fractions!
5⅓
- 3⅚ = ?
Set up your number line the same as before, only this time,
instead of multiples of 10 between the two numbers, use
whole numbers:
3⅚
4
5
5⅓
Then, like before, mark the distance between the points on
the number line:
1/6
3⅚
1
4
1/3
5
5⅓
Now add the distances you marked above the number line. You
still need to find a common denominator, but you are adding
and not subtracting. (Think what the problem would look like if
you found a common denominator first, and then tried to
subtract. You would have to regroup, or borrow, in order to do
this. Much trickier with fractions!)
1/6 + 1 + 2/6 = 1 3/6 or 1 1/2