Again, these are easily done in your head. If the other kids in your class have problems with fractions
they will be really impressed when you just look at a problem and call out the answer.
Test yourself
Try these for yourself (do them in your head):
a) ¼ − 1/5 =
b) ½ − 1/6 =
c) ¼ − 1/7 =
d) 1/3 − 1/7 =
The answers are:
a) 1/20
b) 1/3
c) 3/28
d) 4/21
Can you believe that adding and subtracting fractions could be so easy?Multiplying fractions
What answer would you expect if you had to multiply one-half by two? You would have two halves.
How about multiplying a third by three? You would have three thirds. How much is two halves? How
much is three thirds?
If you split something into halves and bring the two halves together, how much have you got? A
whole. You could say it mathematically like this: ½ × 2 = 1.
If you cut a pie into thirds, and you have all three thirds, you have the whole pie. You could say it
mathematically like this:
1/3 × 3 = 1.
Here is another question: what is half of 12? The answer is obviously 6. But, what did you do to find
your answer? To find half of a number you divide by 2. Or, you could say, you multiplied by one-half. If
you can understand that, you can multiply fractions.
Here is how we do it. We simply multiply the numerators to get the numerator of the answer, and we
multiply the denominators to get the denominator of the answer. Easy!
Let’s try it to find half of 12.
Any whole number can be expressed as that number over (divided by) 1. So 12 is the same as 12/1.
Let’s try another:To calculate the answer we multiply 2 × 4 to get 8. That is the top number of the answer. To get the
bottom number of our answer we multiply the bottom numbers of the fractions.
3 × 5 = 15
The answer is 8/15. It is as easy as that.
What is half of 17?Multiply the numerators.