180 Speed Math for Kids
(^2) – (^1) = 8 – 3 = 5
3 4 12 12
What we did was multiply 2 × 4 = 8, and then subtract 3 × 1 = 3, to
get 5. Five is the numerator of our answer. We multiplied 3 × 4 = 12
to get the denominator. You could do that entirely in your head.
Test you rself
Try these for yourself. Try them without writing anything
down except the answers.
a)^1 / 2 –^1 / 7 =
b)^1 / 3 –^1 / 5 =
c)^1 / 5 –^1 / 7 =
d)^1 / 2 –^1 / 3 =
The answers are:
a)^5 / 14 b)^2 / 15 c)^2 / 35 d)^1 / 6
If you made any mistakes, read the section through again.
Try some for yourself and explain this method to your closest friend.
Tell your friend not to tell the rest of your class.
A shortcut
Just as with addition, there is an easy shortcut when you subtract
fractions when both numerators are 1. Th e only thing to remember
is to subtract backward.
(^1) – (^1) = 5 – 2 (^) = 3
2 5 10 10
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.
cc18.indd Sec1:180 18 .indd Sec 1 : 180 1/9/07 8:40:37 AM 1 / 9 / 07 8 : 40 : 37 AM