Adding in your head

[Miss Chicken]

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

 

[Pondwater]

48
+27
_____
75

 

[StarCruiser]

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).

 

[Miss Chicken]

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

 

[auntiehill]

48
+27
_____
75

This way. But, sadly, I still actually say the words "carry the one" in my head.

 

[Miss Chicken]

yes, adding from right to left and carrying the 1 was another way people explained how they did it.

 

[Pondwater]

This way. But, sadly, I still actually say the words "carry the one" in my head.

I did that same thing.

 

[rhubarbodendron]

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

yep. That's the way I do it

 

[Myko]

I would do 48 + 20 = 68, then add the 7.

 

[Relayer1]

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.

50+30=80, then 80-2-3=75

Or

40+20=60
8+7=15
60+15=75

 

[TEACAKE'S PLEATHER DOME]

40 + 20 = 60
8 + 7 = 15
60 +15 = 75

 

[Tosk]

My head maths is crap...I did it by rounding up. 50 plus 30, minus 5.

 

[Miss Chicken]

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.

 

[Myko]

Sometimes I just wait the 30 seconds until Rachel Riley gives me the answer.

 

[JirinPanthosa]

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.

 

[Silvercrest]

48
+27
_____
75

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.

 

[Kor]

48+20+2+5

Kor

 

[John Clark]

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

That's the way I'd do it.

 

[Spot's Meow]

I would do 48 + 20 = 68, then add the 7.

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.

 

[RandyS]

My head maths is crap...I did it by rounding up. 50 plus 30, minus 5.

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.