ffirs.indd

Multiplication by 11 71
Nine plus 5 is 14. We add 1 to the 9 to get 10. We work with 10 the
same as we would with a single-digit number. Ten is the fi rst part
of our answer, 4 is the middle digit and 5 remains the fi nal digit.
We say, “One thousand and... forty-fi ve.” We could also just say
“Ten... forty-fi ve.”
Practice some problems for yourself and see how quickly you can call
out the answer. You will be surprised how fast you can call them out.
Th is approach also applies to numbers such as 22 and 33. Twenty-two
is 2 times 11. To multiply 17 by 22, you would multiply 17 by 2 to
get 34, then by 11 using the shortcut, to get 374. Easy.
MULTIPLYING LARGER NUMBERS BY 11
Th ere is a very simple way to multiply any number by 11. Let’s take
the example of 123 × 11.
We would write the problem like this:
0123 × 11
We write a zero in front of the number we are multiplying. You will
see why in a moment. Beginning with the units digit, add each digit
to the digit on its right. In this case, add 3 to the digit on its right.
Th ere is no digit on its right, so add nothing:
3 + 0 = 3
Write 3 as the last digit of your answer. Your calculation should look
like this:
0123 × 11
3
Now go to the 2. Th ree is the digit on the right of the 2:
2 + 3 = 5