[pard68]

Neat, I never knew this math trick before! Kind of seems obvious now... 2769=27710-27

And it works for "N1" too, 2771=27710+27

I'm not good enough at mental math to do 277 or 2707 so I don't think this will be too useful to me, but a good trick to know still!

[mkl]

You need to escape those *s with \ or put spaces around them.

This kind of method works much more generally. E.g. 58×33 will be close to 60×30 = 1800, so 58×30 = 1800 - 2×30 = 1740, so 58×33 = 1740+3×58 = 1740+3×60-3×2 = 1740+180-6 = 1940-20-6 = 1914.

Similarly 27×7 = 30×7-3×7 = 210-21 = 189, 270×7 = 10×27×7 = 10×189 = 1890, etc.

[jmholla]

For those confused, pard68 actually meant to write

    27*69=27*7*10-27

which is rounding up one of the numbers and subtracting the multiplicative difference.

[vidarh]

You can decompose it further. 27 times 7 is 20 times 7 plus 7 times 7.

You can also multiply by 100 and 30 and subtract the latter, and then another 27.

In general the method is just about finding which calculations are easy enough to you to decompose the operations into.

[baq]

Another interesting one - 8% of 25 = 25% of 8

[DougN7]

Isn’t that just 8/100 x 25 = 25/100 x 8 = (8 x 25)/100