Multiplication Using Two Reference Numbers 99
How about 7 × 26? I would say 20 times 7 is 140. To multiply by
20 we multiply by 2 and by 10. Two times 7 is 14, times 10 is 140.
Th en 7 times 6 is 42. Th e answer is 140 plus 42, which is 182.
Experiment for yourself to see which method you fi nd easiest. You
will fi nd you can easily do the calculations entirely in your head.
PLAYING WITH TWO REFERENCE NUMBERS
Let’s multiply 13 times 76. We would use reference numbers of 10
and 70. We wouldn’t normally use 80 as a reference number because
we would have a subtraction at the end. Let’s try a few methods to
see how they work.
First, using 10 and 70.
+3
+6
(10 × 7) 13 × 76 =
We multiply the 3 above the 13 by the multiplication factor of 7 in
the parentheses. Th ree times 7 is 21; write 21 in a circle above the 3.
Now we add crossways.
76 + 21 = 97
Multiply 97 by the base reference number of 10 to get 970. Now
multiply 3 times 6 to get 18. Add this to 970 to get 988.
Here is the full calculation:
+21
+3
+6
(10 × 7) 13 × 76 = 970
- 18
988 Answer