A handful of shortcuts turn slow multiplications into one step. They come up again and again in speed tests, and the trading-style arithmetic screens lean on them.
- A number ending in 5, squared: multiply the tens digit by the next number up, then write 25. 35 × 35: 3 × 4 = 12, so 1,225.
- Times 11 for two-digit numbers: write the first digit, then the sum of both digits, then the last digit. 43 × 11 = 4, 7, 3 = 473. If the middle sum is 10 or more, carry the 1.
- Times 25 is times 100, then divide by 4. Dividing by 25 is times 4, then divide by 100.
- Powers of ten move the decimal point: × 100 moves it two places right, ÷ 1,000 three places left.
With decimals, multiply as if they were whole numbers, then count the decimal places in the question and put the same number in the answer: 0.3 × 0.2 is 3 × 2 = 6 with two places, 0.06.