Multiplication Using Two Reference Numbers 101
Th irteen is 3 above 10, and 76 is 1 above 75.
We multiply the 3 above the 13 by 7½. Is that diffi cult? No, we
multiply 3 by 7 to get 21, plus half of 3 to get another 1½.
21 + 1½ = 22½
Now we add crossways.
76 + 22½ = 98½
Now we multiply 98½ by the base reference number of 10 to get
- Th en we multiply the numbers in the circles.
3 × 1 = 3
Th en we add 985 and 3 to get our answer.
985 + 3 = 988 Answer
Th e complete calculation looks like this:
+3
+1
(10 × 7½) 13 × 76 = 985
- 3
988 Answer
Th is is something to play with and experiment with. We just used
the same formula three diff erent ways to get the same answer. Th e
last method (using a fraction as a multiplication factor) can be used
to make many multiplication problems easier.
Let’s try 96 times 321. We could use 100 and 325 as reference
numbers.
(100 × 3¼) 96 × 321 =
–4
–4
+22 +22^11 // 22