Checking answers
Let us apply what we learnt in the last chapter and check our answer:
The substitute numbers for 23 and 21 are 5 and 3.
5 × 3 = 15
1 + 5 = 6
Six is our check answer.
The digits in our original answer, 483, add up to 6:
4 + 8 + 3 = 15
1 + 5 = 6
This is the same as our check answer, so we were right.
Let’s try another:
23 × 31 =
We put 3 and 11 above 23 and 31:They are 3 and 11 above the reference number, 20. Adding diagonally, we get 34:
31 + 3 = 34
or
23 + 11 = 34
We multiply this answer by the reference number, 20. To do this, we multiply 34 by 2, then multiply
by 10.
34 × 2 = 68
68 × 10 = 680
This is our subtotal. We now multiply the numbers in the circles (3 and 11):
3 × 11 = 33
Add this to 680:
680 + 33 = 713
The calculation will look like this: