Subtracting Mixed Numbers with like fractional parts
This is a free 5th grade lesson about subtracting mixed numbers with like fractional parts (no need to convert the fractions to have a common denominator). The lesson presents two strategies for subtraction: regrouping and
subtracting in parts (piece by piece). The next lesson presents yet a third strategy (using a negative fraction).
Strategy 1: Renaming /
regrouping
In this method you
divide one whole pie into “slices,” and join these slices
with the existing slices. After that, you can subtract. It is the same
as
regrouping in the subtraction of whole numbers.
Rename 3
2
6
as 2
8
6
, and then subtract 1
5
6
.
At first we have three uncut pies and 2/6 more. Then
we cut one of the whole pies into
sixths. We end
up with only two whole (uncut) pies and 8 sixths.
We say that 3 2/6 has been renamed as 2 8/6.
Now we can subtract 1 5/6 easily.
3
2
6
− 1
5
6
=
2
8
6
− 1
5
6
= 1
3
6
Regroup 1 whole pie as 6
sixths.
Regroup
(borrow)1 whole pie
as 6 sixths.
There
were already 2 sixths
in
the
fractional
parts column,
so
that is why it becomes
8/6 after
regrouping.
Now you can
subtract
the 5/6.
Rename 2
1
8
as 1
9
8
, and then subtract
5
8
.
At first we have two uncut pies and 1/8 more. Then we
cut one of the whole pies into eighths. We end up
with only one whole (uncut) pie and 9 eighths.
We say that 2 1/8 has been renamed as 1 9/8.
Then we can subtract 5/8 easily.
2
1
8
−
5
8
=
1
9
8
−
5
8
= 1
4
8
Regroup 1 whole pie as 8
eighths.
Regroup (borrow)1 whole pie
as 8 eighths.
Since
there was already one
eighth in the fractional parts
column,
it
becomes
9/8 after
regrouping.
Now
you can
subtract 5/8.
1. Do not
subtract anything. Just cut up one whole pie into fractional parts. Rename the mixed number.
a. 2
1
6
is renamed as
b. 3
1
8
is renamed as
c. 2
2
9
is renamed as
d. 2
3
5
is renamed as
e. 3
3
10
is renamed as
f. 2
1
4
is renamed as
2. Rename, then subtract. Be
careful. Use the pie pictures to check your calculation.
a.
4
2
9
− 1
8
9
↓
=
3
11
9
− 1
8
9
=
b.
5
3
12
− 2
7
12
↓
=
4
12
− 2
7
12
=
c.
5
7
10
− 3
9
10
↓
=
− 3
9
10
=
d.
4
3
8
− 1
7
8
↓
=
− 1
7
8
=
3. Regroup (if needed) and subtract.
a.
b.
7
4
9
− 2
7
9
c.
12
9
12
− 6
11
12
d.
8
3
14
− 5
9
14
e.
14
7
9
− 3
5
9
f.
11
5
21
− 7
15
21
g.
26
4
19
− 14
15
19
h.
10
3
20
− 5
7
20
Strategy 2: Subtract in Parts
First, subtract what you can from the
fractional part of the minuend. Thensubtract the rest from
one of the whole
pies. The examples show two slightly different ways to understand this.
2
1
8
−
5
8
= ?
We cannot subtract 5/8
from 1/8. So, first
subtract
1/8, which leaves
2 whole pies.
The rest
(4/8) of
the 5/8 is
subtracted
from one of the whole pies.
2
1
8
−
5
8
=
2
1
8
−
1
8
−
4
8
=
=
2
−
4
8
= 1
4
8
3
2
9
− 2
7
9
= ?
We cannot subtract 7/9
from 2/9.
So, first subtract
2 2/9, which
leaves
1 whole pie. The rest (5/9)
is
subtracted
from the last whole pie.
3
2
9
− 2
7
9
=
3
2
9
− 2
2
9
−
5
9
=
=
1
−
5
9
=
4
9
4. Subtract in parts. Remember:
you can add to check a subtraction problem.
a. 2
2
6
−
5
6
=
b. 3
1
5
− 2
3
5
=
c. 3
1
8
−
1
7
8
=
d. 3
2
7
−
2
6
7
=
e. 5
2
9
−
5
9
− 1
8
9
=
5. Subtract in two parts. Write
a subtraction sentence.
a.
Cross out
3
4
.
b.
Cross out 1
5
7
.
c.
Cross out 1
5
9
.
d.
Cross out 1
11
12
.
Example. Look at Mia’s math work: 7
1
6
− 2
5
6
= 9
6
6
= 10. Can you see why it is
wrong?
If you have 7 and a bit and you subtract
2 and some, you cannot get 10 as an answer! In reality, Mia was
adding
instead of subtracting. (If you have ever done that, you are not
alone—it is a common error.)
Always check if your answer is
reasonable.
6. Subtract.
a. 8
1
5
−
3
3
5
=
b. 4
2
8
− 1
7
8
=
c. 12
4
13
− 9
8
13
=
d. 11
2
15
−
6
6
15
=
e. 7
1
20
− 3
7
20
=
f. 6
14
100
− 2
29
100
=
7. Two sides of a triangle measure 3 5/8 in., and the perimeter of
the triangle is 10 1/8 in. How long is the third side of the
triangle?
8. Ellie had 4 yards of material. She needed 7/8 yard for making a skirt,
and
she made two. How much material is left?
9. Harry wants to bake chocolate chip cookies. The recipe calls for 1 3/4
cups of flour
and he is making a double batch. However, Harry only
has 3/4 cup of flour!
How much more flour would Harry need to borrow from his
neighbor?
A self-teaching worktext for 5th grade that teaches fractions and their operations with visual models. The book covers fractions, mixed numbers, adding and subtracting like fractions, adding and subtracting mixed numbers, adding and subtracting unlike fractions, and comparing fractions.
Download ($3.50). Also available as a printed copy.