In this lesson we solve fraction divisions by thinking how many times the divisor "fits" or "goes into" the dividend. For example, the fraction 1/4 goes into five 20 times, so 5 ÷ (1/4) = 20. The lesson has lots of exercises with visual models and many word problems. The previous lesson had to do with dividing fractions by whole numbers.
In the video, I explain two different division situations where we don't have to use the "rule" or shortcut for fraction division, but instead can use mental math. The first is when a fraction is divided by a whole number. The second is when the answer to a fraction division is a whole number.
How many times does one thing fit into another?You can always write a division from this situation.
Think: “How many times does the divisor go
into the dividend?”
How many times does
go into
?
Eight times. We can write a division: 2 ÷
1
4
= 8.
Then check the division: 8 ×
1
4
=
8
4
= 2.
How many times does
1
2
go into
3?
Six times. We can write a division: 3 ÷
1
2
= 6.
Then check the division: 6 ×
1
2
=
6
2
= 3.
1. Solve. Write a
division. Then write a multiplication that checks your division.
a. How many times does
go into
?
2 ÷
1
3
= _____
Check: ____ ×
1
3
=
b. How many times does
go into
?
1 ÷
1
4
= _____
Check: ____ ×
1
4
=
c. How many times does
go into
?
6 ÷
1
3
= ____
Check:
d. How many times does
go into
?
5 ÷
1
4
= ____
Check:
Now you write the division. Be careful: the
divisor is the number that “goes into” the dividend.
e. How many times does
go into
?
____
÷
=
Check:
f. How many times does
go into
?
____
÷
____
=
Check:
g. How many times does
1
6
go into 2?
____
÷
____
=
h. How many times does
1
5
go into 3?
____
÷
____
=
2. Divide. Think, “How many times does
the divisor go into the dividend?” Use the pictures to help.
a. 3 ÷
1
6
=
b. 4 ÷
1
9
=
c. 4 ÷
1
8
=
d. 3 ÷
1
2
=
e. 3 ÷
1
7
=
f. 4 ÷
1
5
=
g. 2 ÷
1
3
=
Did you notice a pattern? There is a shortcut
to dividing a whole number by a unit fraction!
5
÷
1
4
↓
↓
5
×
4
= 20
3
÷
1
8
↓
↓
3
×
8
= 24
9
÷
1
7
↓
↓
9
×
7
= 63
Why does it work that way? For example, consider the problem 5 ÷ (1/4). Since 1/4 goes
into 1 exactly 4 times, it must go into 5 exactly 5 × 4 = 20
times.
3. Solve. Use the
shortcut.
a. 3 ÷
1
6
=
b. 4 ÷
1
5
=
c. 3 ÷
1
10
=
d. 5 ÷
1
10
=
e. 7 ÷
1
4
=
f. 4 ÷
1
8
=
g. 4 ÷
1
10
=
h. 9 ÷
1
8
=
4. Write a division for each
word problem, and solve. Do not write just the answer.
a. How many 1/2-meter pieces can
you cut from
a roll of string that is 6 meters long?
b. How many 1/4-cup
servings can you get from 2 cups of almonds?
c. Ben has small weights that weigh 1/10 kg each. How many
of those would he need to make 5 kg?
d. An eraser is 1/8 inches thick. How many
erasers can be stacked into a
4-inch tall box?
5. Write a story problem to match each division, and solve.
a. 2÷
1
2
=
b. 5÷
1
3
=
6. These divisions are not as easy as the previous
ones, but they are not difficult either. Again, think
how many times the divisor goes into the dividend. The pictures
can help.
a. 4 ÷
2
3
=
b. 4 ÷
4
5
=
c. 2
5
6
÷
1
6
=
d. 3 ÷
6
10
=
e. 3
5
9
÷
4
9
=
f. 2
4
8
÷
5
8
=
7. Write a division and solve. Write also a
multiplication to check your division.
a. How many times does
go into
?
____
÷
____
=
____
____
×
____
=
____
b. How many times does
go into
?
____
÷
____
=
____
____
×
____
=
____
c. How many times does
go into
?
____
÷
____
=
____
____
×
____
=
____
d. How many times does
go into
?
____
÷
____
=
____
____
×
____
=
____
8. A recipe calls for 1/2 cup of
butter, among other ingredients.
Alison had plenty of all of the other ingredients except the butter.
How many batches of the recipe can she make if she
has ...
a. 3 cups of butter?
b. 2
½ cups of butter?
9. Jackie made three apple pies
and divided them into twelfths.
She plans on serving two slices to each guest. How many
servings will she get out of the three pies? Hint: Draw a picture.
10. How many 2/10-liter servings
do you get from 1 liter of juice?
Out of 4 liters of juice?
11. When
Natalie goes jogging, she jogs for 1/4 mile, then walks for 1/4 mile,
then again jogs for 1/4 mile, and so on. How many
such stretches
are there
for her in a jogging track that is 2 1/2 miles long?
12. Jill makes bead necklaces that must be
exactly 24 inches long. She has size
SS beads, which are 1/8-inch thick, and size S
beads, which are 1/4-inch thick.
Bead
Width
SS
1/8 in
S
1/4 in
a. How many beads
would be in a necklace
made solely of SS beads?
b. How many beads
would be in a necklace
made solely of S beads?
c. She also makes a
necklace with the pattern SS-S-SS-S.
How many of each kind of bead does she need?
A self-teaching worktext that teaches fractions using visual models, a sequel to Math Mammoth Fractions 1. The book covers simplifying fractions, multiplication and division of fractions and mixed numbers, converting fractions to decimals, and ratios.
Download ($5.75). Also available as a printed copy.