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?
Sums
20+40= 60Tichtheid 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?
8+7 = 15
60+15= 75
Edit. Would do the same as you for 2nd sum.
Ian Madigan for Ireland.
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inactionman
- Posts: 3398
- Joined: Tue Jun 30, 2020 7:37 am
slightly different for addition: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?
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)
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Insane_Homer
- Posts: 5529
- Joined: Tue Jun 30, 2020 3:14 pm
- Location: Leafy Surrey
This
“Facts are meaningless. You could use facts to prove anything that's even remotely true.”
FirstTichtheid 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?
48+7=55
55+20=75
2nd
10×78=780
3×75=225
780+225=1005
3×3=9
1005+9=1014
