Multiplication · level 1
Just above 100
Above the reference number the circles are positive, so you cross-add instead of cross-subtract. Nothing else changes.
Worked example: 106 × 104
Step by step
- 1
Choose the reference number: 100
106 is + 6 · 104 is + 4
Write each number with its difference from the reference in a circle.
- 2
Cross-subtract (or cross-add) diagonally
106 + 4 = 110
The other diagonal gives the same answer: 104 + 6 = 110.
- 3
Multiply that result by the reference number
110 × 100 = 11,000
- 4
Multiply the two circled numbers
(+6) × (+4) = 24
- 5
Add it to the running total
11,000 + 24 = 11,024
106 × 104 = 11,024
The theory behind it
Intuition
Multiplication is easiest near a round number. You multiply the friendly number, then repair the difference — the repair term is always (how far off) × (the other factor).
Common pitfalls
- ×Losing the sign of the correction: rounding up means you subtract, rounding down means you add.
- ×Correcting against the wrong factor — the repair multiplies the factor you did not round.
In the interview
Every case round is a multiplication: users × price, volume × margin, market × share. Speed here buys you thinking time for the so-what.