1. Quick Square
If you need to square a 2-digit number ending in 5, multiply the first digit by itself plus 1, and put 25 on the end.
252 = (2x(2+1)) & 25
2 x 3 = 6
625
2. Multiplying by 9
To multiply any number between 1 and 9 by 9, hold both hands in front of your face, with fingers extended. Now drop the finger that corresponds to the number you are multiplying (for example, for 9 x 3, drop your third finger). Now count the fingers before the dropped finger (in the case of 9 x 3 it is 2) — that‘s your first digit. Then, count the fingers after (again in this case, it’s 7). The answer is 27.
3. Dividing by 5
To divide a large number by five, all you need to do is multiply by 2 and move the decimal point one space to the left:
195 / 5
Step 1: 195 x 2 = 390
Step 2: Move the decimal left; 39.0, or just 39
2978 / 5
Step 1: 2978 x 2 = 5956
Step 2: 595.6
4. Subtracting from 1,000
To subtract a large number from 1,000, subtract all but the last number from 9, then subtract the last number from 10:
1000 – 648 = ?
Step 1: subtract 6 from 9 = 3
Step 2: subtract 4 from 9 = 5
Step 3: subtract 8 from 10 = 2
Answer: 352
