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!
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.
27*69=27*7*10-27
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.