More Mental Math Tricks

Multiplication tricks

When multiplying two numbers, you can sometimes simplify the process by doubling one number and halving the other. This trick works well when one number is even. For example, to multiply 16 by 25:

1. Halve 16 and double 25.
   • 16×25=8×50
2. Multiply the new numbers together.
   • 8×50=400

So, 16×25=400.

Multiplying by 5

Multiplying by 5 can be simplified using this method:

1. Divide the number by 2.
   • If you need to multiply 246 by 5, first divide 246 by 2, 246÷2=123.
2. Multiply the result by 10.
   • 123×10=1230

So, 246×5=1230.

Divisibility by 3

To quickly check if a number is divisible by 3, simply add up all the digits of the number. If the sum is divisible by 3, then the original number is too. For example, check if 852 is divisible by 3:

1. Add the digits together.
   • 8+5+2=15
2. Check if the sum is divisible by 3.
   • Since 15 is divisible by 3, so is 852.

Subtracting from 10^x

Subtracting large numbers from 1000 can be done quickly with this trick:

1. Subtract each digit of the number from 9, except the last digit.
   • For example, to subtract 357 from 1000:
   • Subtract the first digit from 9: 9−3=6
   • Subtract the second digit from 9: 9−5=4
   • Subtract the last digit from 10: 10−7=3
2. Combine the results.
   • The answer is 643

So, 1000−357=643

Rounding 

Adding large numbers mentally can be easier by breaking them down:

1. Round up or down to the nearest ten or hundred.
   • To add 678 and 495, round 678 up to 680 and 495 up to 500.
2. Add the rounded numbers.
   • 680+500=1180
3. Subtract the difference caused by rounding.
   • 1180−(2+5)=1180−7=1173

So, 678+495=1173.