If your friend has one-quarter of a pie, and she gives you half, how much of the pie do you have? Or, to put it another way, what's half of one-quarter? Or, to put it into mathematical notation:
1/2 x 1/4 = ?
To get the answer, multiply the numerators (the top parts) and denominators (the bottom parts) separately.
In this case, first we multiply the numerators:
1 x 1 = 1
Next we multiply the denominators:
2 x 4 = 8
The answer has a numerator of 1 and a denominator of 8. In other words:
1/2 x 1/4 = 1 x 1/2 x 4 = 1/8
You have one-eighth of the pie.
Another Example
Let's try another.
2/9 x 3/4 = ?
First we multiply the numerators:
2 x 3 = 6
Next we multiply the denominators:
9 x 4 = 36
The answer has a numerator of 6 and a denominator of 36. In other words:
2/9 x 3/4 = 2 x 3/9 x 4 = 6/36
This can be further reduced:
6 ÷ 6/36 ÷ 6 = 1/6
(See Reducing Fractions.)
Multiplying Mixed Numbers
To multiply two mixed numbers, or a mixed number and a fraction, first convert each mixed number to a fraction. Then multiply the fractions.
What is 21/3 x 1/4 = ?
First we write 21/3 as a fraction:
21/3 = 7/3
Then we multiply the fractions.
7/3 x 1/4 = ?
First we multiply the numerators:
7 x 1 = 7
Next we multiply the denominators:
3 x 4 = 12
The answer has a numerator of 7 and a denominator of 12. In other words:
21/3 x 1/4 = 7 x 1/3 x 4 = 7/12