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General Discussion & Feedback => Just Chat! => Topic started by: scientizscht on 09/04/2019 18:55:38

Title: How to do 756 x 675 mentally?
Post by: scientizscht on 09/04/2019 18:55:38

Hello!

How can you do e.g. 756 x 675 mentally?

Any good trick?

Title: Re: How to do 756 x 675 mentally?
Post by: Bored chemist on 09/04/2019 20:42:39

It's roughly 700 *800 which is 560,000.
756 is about 1/16 short of 800 which is about 7% less
675 is 25 / 700 short
So that's 27/7 % low- call it 4%

so the product will be about 11% less than 560000
That's about 500,000

My calculator confirms that's within 3% of the right answer.
That's probably good enough for a lot of things
If not I can get a calculator or a slide rule or a pencil + paper.

Title: Re: How to do 756 x 675 mentally?
Post by: syhprum on 10/04/2019 02:23:50

How you do it depends on what you need the answer for, if it is for something important use a written form or a calculator a quick assessment mentally would be that it is 3/4 of 675000 so subtract 170000 from 675000 which is 505000 as against the correct value of 510300 98.96% of the correct value

Title: Re: How to do 756 x 675 mentally?
Post by: syhprum on 10/04/2019 02:27:47

How strange that slide rules hang around my son when he joined technical school in 1973 was asked to provide a slide rule long after cheap calculators became available.