ffirs.indd

144 Speed Math for Kids
A good general rule for dividing by factors is to divide by the smaller
number fi rst and then by the larger number. Th e idea is that you
will have a smaller number to divide when it is time to divide by the
larger number.
Dividing by numbers such as 14 and 16 should be easy to do
mentally. It is easy to halve a number before dividing by a factor. If
you had to divide 368 by 16 mentally, you would say, “Half of 36 is
18, half of 8 is 4.” You have a subtotal of 184. It is easy to keep track
of this as you divide by 8.
Eighteen divided by 8 is 2 with 2 remainder. Th e 2 carries to the
fi nal digit of the number, 4, giving 24. Twenty-four divided by 8 is
exactly 3. Th e answer is 23 with no remainder. Th is can be easily
done in your head.
If you had to divide 2,247 by 21, you would divide by 3 fi rst, then
by 7. By the time you divide by 7, you have a smaller number to
work with.
2,247 ÷ 3 = 749
749 ÷ 7 = 107
It is easier to divide 749 by 7 than to divide 2,247 by 7.
Division by numbers ending in 5
To divide by a two-digit number ending in 5, double both numbers
and use factors. As long as you double both numbers, the answer
doesn’t change. Th ink of 4 divided by 2. Th e answer is 2. Now double
both numbers. It becomes 8 divided by 4. Th e answer remains the
same. (Th is is why you can cancel fractions without changing the
answer.)
Let’s have a try:
1,120 ÷ 35 =