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Sums
Posted: Fri Jul 21, 2023 8:18 am
by Tichtheid
I saw a tweet, these come up all the time on SM and I never bother answering them but this one intrigued me as there are many different ways to do these. The question was "What happens in your head when you see "27 + 48 = ?"
I would do
27 + 40 = 67.
67 + 8 = 75.
Similarly if I see 13 x 78 I'd do
10 x 78 = 780
3 x 78 = (3 x 80 - 6) = 234
780 + 234 = 980 + 34 = 1014
How does your head work?
Re: Sums
Posted: Fri Jul 21, 2023 8:20 am
by Jim Lahey
Tichtheid wrote: Fri Jul 21, 2023 8:18 am
I saw a tweet, these come up all the time on SM and I never bother answering them but this one intrigued me as there are many different ways to do these. The question was "What happens in your head when you see "27 + 48 = ?"
I would do
27 + 40 = 67.
67 + 8 = 75.
Similarly if I see 13 x 78 I'd do
10 x 78 = 780
3 x 78 = (3 x 80 - 6) = 234
780 + 234 = 980 + 34 = 1014
How does your head work?
20+40= 60
8+7 = 15
60+15= 75
Edit. Would do the same as you for 2nd sum.
Re: Sums
Posted: Fri Jul 21, 2023 8:26 am
by inactionman
Tichtheid wrote: Fri Jul 21, 2023 8:18 am
I saw a tweet, these come up all the time on SM and I never bother answering them but this one intrigued me as there are many different ways to do these. The question was "What happens in your head when you see "27 + 48 = ?"
I would do
27 + 40 = 67.
67 + 8 = 75.
Similarly if I see 13 x 78 I'd do
10 x 78 = 780
3 x 78 = (3 x 80 - 6) = 234
780 + 234 = 980 + 34 = 1014
How does your head work?
slightly different for addition:
7 + 8 = 15
20 + 40 = 60
add together, either directly or after rounding up units of ten.
I definitely work in units of ten when multiplying as well - so pretty much same as you. It's just easier to keep track of, if nothing else.
e.g. 98 * 4 is easier worked as (100 * 4) - (2 * 4)
Re: Sums
Posted: Fri Jul 21, 2023 8:32 am
by Ymx
I’d first do
7+8=10 + 5
Then do
20+40+10 = 70
70+5=75
Basically just long addition in my head.
Re: Sums
Posted: Fri Jul 21, 2023 8:34 am
by Ymx
10x78 +
3 x 70 + 3 x 8
= 990 + 24
= 1014
Re: Sums
Posted: Fri Jul 21, 2023 10:47 am
by PornDog
27+48 =
7+48 = 55 +20 = 75
13*78 =
12*8 = 96
12*70 = 840
96+840+78 (840+100 = 940 + 80 = 1020 -4 -2) = 1014
Addition I'm perfectly happy with, but I'm sure the multiplication is overly cumbersome
Re: Sums
Posted: Fri Jul 21, 2023 10:51 am
by Insane_Homer
Jim Lahey wrote: Fri Jul 21, 2023 8:20 am
20+40= 60
8+7 = 15
60+15= 75
This
Re: Sums
Posted: Fri Jul 21, 2023 11:15 am
by Sandstorm
27 + 50 = 77
77 - 2 = 75
Re: Sums
Posted: Fri Jul 21, 2023 11:50 am
by Slick
I'd do the addition the same as the OP
I wouldn't even attempt the second one
Re: Sums
Posted: Fri Jul 21, 2023 12:15 pm
by TB63
30 + 50 - 5..
Re: Sums
Posted: Fri Jul 21, 2023 5:26 pm
by petej
Tichtheid wrote: Fri Jul 21, 2023 8:18 am
I saw a tweet, these come up all the time on SM and I never bother answering them but this one intrigued me as there are many different ways to do these. The question was "What happens in your head when you see "27 + 48 = ?"
I would do
27 + 40 = 67.
67 + 8 = 75.
Similarly if I see 13 x 78 I'd do
10 x 78 = 780
3 x 78 = (3 x 80 - 6) = 234
780 + 234 = 980 + 34 = 1014
How does your head work?
First
48+7=55
55+20=75
2nd
10×78=780
3×75=225
780+225=1005
3×3=9
1005+9=1014
Re: Sums
Posted: Fri Jul 21, 2023 7:46 pm
by Torquemada 1420
I just see 75.