1 × 4 = 4
680 + 4 = 684 Answer
The base number is always 10 when you multiply by 9. Let’s try another.
9 × 486 =
The reference numbers are 10 and 490. You always go up to the next 10 for the second reference
number. We write it like this:
We subtract 49 from 486. Is this difficult? Not at all. We subtract 50 and add 1:
486 − 50 = 436, plus 1 is 437
Multiply the numbers in the circles.
1 × 4 = 4
This result is always the final digit in the answer when you multiply by 9. The answer is 4,374. The
completed problem looks like this:
The problems are easy to calculate in your head.Using fractions as multiples
There is yet another possibility. The multiple can be expressed as a fraction.
What do I mean? Let’s try 94 × 345. We can use 100 and 350 as reference numbers, which we would
write as (100 × 3½) . Our problem would look like this:
Here we have to multiply-6 by 3½. To do this, simply multiply 6 by 3, which is 18, then add half of 6,
which is 3, to get 21.
Subtract 21 from 345. Multiply the result by 100, then multiply the numbers in the circles, and then
add these two results for our answer.
345 − 21 = 324
324 × 100 = 32,400
6 × 5 = 30
32,400 + 30 = 32,430 Answer
The completed problem looks like this: