You probably havent had to do longhand math in years, but you do mental math every day.
Here are some shortcuts that will help you do more math in your head.
So 68% of 25 = 25% of 68 = 68/4 = 17.
If the second number has some bigger digits than the first, it gets more complicated.
To avoid borrowing places, you want to get rid of those bigger digits.
Heres how:
Say youre calculating 925-734.
That tens place makes things a little complicated.
Multiples of 4: Ignore everything from the hundreds place up.
Divide the remaining two-digit number in half.
Then execute the multiples-of-2 test.
All (and only) multiples of 5 end in 5 or 0.
Multiples of 6: initiate the 2 test and the 3 test.
Multiples of 7: There area few tests, but theyre all harder than digging out your phone.
This ones probably the easiest:
Double the units and subtract from the tens.
E.g., 1365136(25)=12612(26)=0.
Multiples of 8: Ignore everything from the thousands place up.
Divide the remaining three-digit number in half.
Then in half again.
Then initiate the multiples-of-2 test.
All (and only) multiples of 10 end in 0.
For example, 60 = 223*5.
So all multiples of 60 are also multiples of 2*2, 3, and 5.
(150 is divisible by 2, but not by 4, so its not divisible by 60.)
For example:
Doubling numbers tends to be easier.
So when multiplying by an even number, first multiply by half that number, then by 2.
Multiply by 5: First multiply by 10, then divide by 2.
Multiply by 9: Multiply by 10 and subtract the number.
So 659 = (6510)-65 = 650-65 = 585.
Multiply a single-digit numberxby 9: The first digit isx-1.
The second digit is 9 minus the first digit.
So 8*9=72.
If youve forgotten your times tables,brush up on them.
For example, you know that 102 is 100.
So 112 = 100+10+11, or 121.
And 122 = 121+11+12 = 144.
And 132 = 144+12+13 = 169.
To square a two-digit number, round it first
Say you gotta square 46.
So 462 = (50*42)+42 = 2,100+16 = 2,116.
Combining mental math tricks really increases your power.
If you didnt follow that, heresa longer explanation that might do the trick.
Convert temperatures
To roughly convert from Celsius to Fahrenheit, multiply by 2 and add 30.
From Fahrenheit to Celsius, subtract 30 and divide by 2.
(To more precisely convert C to F, multiply by 1.8 and add 32.)
The order is important: The addition/subtraction is always closer to the Fahrenheit side of the conversion.
Double your hourly rate and add three zeroes.
So $25/hour is about $50,000/year.
Itll be two weeks low, if you get paid for every weekday of the year.
Thatll be just two and a half workdays over your 52-week salary.
Add that to the rough estimate and youve got your 52-week salary.
But I thought you were here formentalmath.
Find more shortcuts
Listverse has someeasy mental math shortcuts.
Wikipedia has manyadvanced shortcutsthat cover arithmetic, squares and cubes, roots, and logarithms.
And Better Explained lists somecommon unit conversions, like MPH = feet per second*1.5.
