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# Mental Arithmetic

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Time how long it takes you to get an answer to the following subtraction problem, in your head.   No writing stuff down, or using a calculator.

416 - 327 =

(A) What number did you get as the answer?

(B) How long did it take you?

(C) How did you get it?

So, for example, you might reply:

(A) 42

(B) 10 seconds

(C) I counted onwards in 10s, then in 1s.

by on Mar. 23, 2014 at 6:05 AM
Replies (11-20):
by Gold Member on Mar. 23, 2014 at 6:06 AM

This is a spin-off from the thread:

# More Common Core Math Crapmatics

I'm predicting that nobody will get the correct answer in under 10 seconds, using the traditional formal method used for written mathematics:

3    10    16
4  1  6

3  2  7
-----------

0  8  9
-----------

but that some will get the correct answer in under 10 seconds, by using the sort of method that the common core wants taught in addition to the formal method.

by on Mar. 23, 2014 at 6:13 AM
187
30 seconds
I wrote it down in my head like I would on paper.
by on Mar. 23, 2014 at 6:14 AM
Math has always been hard
by Gold Member on Mar. 23, 2014 at 6:16 AM
89. It took me less than ten seconds. I have always been good at mental math - I just looked at the problem the way I would on a piece of paper. I do mental equations every single day at work.
by on Mar. 23, 2014 at 6:58 AM
I am trying to figure out this new math. With the extra steps. And how do you solve this problem under 10 seconds?

Quoting Ashgambit: 89. It took me less than ten seconds. I have always been good at mental math - I just looked at the problem the way I would on a piece of paper. I do mental equations every single day at work.
by on Mar. 23, 2014 at 8:47 AM

89

a few seconds

saw that it wasn't an even 100 difference and that the smaller numbers last two digits were larger than the bigger numbers last two digits by 11..

by on Mar. 23, 2014 at 8:59 AM
89 in 15 seconds.

DD is 8, and got it in 25ish using traditional math (which is what I teach the kids), in her head.
by Platinum Member on Mar. 23, 2014 at 9:12 AM

A) 89

B) less time than its taking to type answer

C) 16 -7 =9  10-2 = 8     89

by Silver Member on Mar. 23, 2014 at 9:16 AM
89 in under 10 using traditional method. But as a disclaimer I work in accounting and do mental math every day with larger numbers.
by Bronze Member on Mar. 23, 2014 at 9:26 AM

a) 89

b) 3 seconds

c) left to right subtraction algorithm. Subtract left to right, rounding while keeping track of how much I rounded to add at the end.