This question is going around Twitter with many different answers so I though I would ask here How do you add up 48 + 27 in your head. Myself I did it this way 8 + 7 = 15 40 + 20 = 60 60 + 15 = 75
Breaking it into manageable chunks is always a good way to handle odd numbers... I've done that with percentages (to a point, of course).
Other people said they rounded up, for example one way mentioned was 50 + 27 = 77 and then subtracted what they had rounded up by 77 - 2 = 75
I use the following method for multiplying in my head. I am not very good at explaining it but I hope this makes sense Though now I only use this method for multiplying a 2 digit number by another 2 digit number when I was younger I could do it with larger numbers.
For that question my thought process was 50 + 27 - 2. For multiplication of two digit numbers my thought process is difference of squares. That is (a - b)(a + b) = a^2 - b^2. I learned a trick on Square One when I was 7 that you can square numbers ending in 5 by doing n times n + 1 then tacking on a 25. So, say 38 times 74. 56 + 18 times 56 - 18. 55 squared = 3025, plus 55 * 2 + 1 = 3136, minus 18 squared (324) = 2812. When I’m doing arithmetic in my head as opposed to on paper I use shortcut heuristics.
Well, that's just cheating! For me it's 48 + 7 = 55 55 + 20 = 75 I think sometimes I'd break it up as 48 + 20 = 68 and 68 + 7 = 75, but I'm not sure.
Same for me, 48 + 20 = 68, then I add on 7 (using my fingers of course). This is pretty much how I add most numbers, take the original number and then make the second number a nice round one, on a ten or a 5, then tack on whatever is left over.
My math is crap period. They say the old joke is 1+1=11, but to me, that's no joke. 2 1's together makes 11. Ironically math is the one subject in school where I got consistent grades. Straight F's every time I took the class.