178 Speed Math for Kids
divide the top number by the bottom, and express the remainder as
a fraction. So, 12 divides one time into 17 with 5 remainder. Our
answer is written like this:
15 / 12
Test you rself
Try these for yourself:
a)^1 / 4 +^1 / 3 =
b)^2 /^5 +^1 /^4 =
c)^3 / 4 +^1 / 5 =
d)^1 / 4 +^3 / 5 =
How did you do? The answers are:
a)^7 / 12 b)^13 / 20 c)^19 / 20 d)^17 / 20
If your math teacher says you must use the lowest common denominator
to add fractions, is there an easy way to fi nd it? Yes, you simply multiply
the denominators and then see if the answer can be simplifi ed. If you
multiply the denominators, you certainly have a common denominator,
even if it isn’t the lowest. (You should understand the standard method
of adding and subtracting fractions as well as the methods taught in this
book.)
A shortcut
Here is a shortcut. If the numerators are both 1, we add the
denominators to fi nd the numerator of the answer (top number),
and we multiply the denominators to fi nd the denominator of the
answer. Th ese can easily be done in your head.
Here is an example:(^1) + (^1) = 4 + 5 (^) = 9
4 5 4 × 5 20
cc18.indd Sec1:178 18 .indd Sec 1 : 178 1/9/07 8:40:36 AM 1 / 9 / 07 8 : 40 : 36 AM