^That is interesting, never heard of that. Found it though.
This article here explains the situation well :
http://www.patheos.com/blogs/friendlyatheist/2015/10/22/sometimes-estimating-is-better-than-getting-the-exact-answer/Long story short, it is about estimating to avoid the exact calculation, so the 'reasonable' answer is the one you get doing the calculation on a larger scale.
The exact calculation is 100-25 = 75
The rough calculation is "around 100"-"around 30" = 70.
The meaning of the whole thing is to learn to do good estimations WITHOUT doing the exact calculation.
I agree with the article editor that the 25-100 pages example is not a good one to demonstrate the theory. Because 100-25 is already a pretty simple math core func.
103-28 is better.
Best demo is the one he uses about the prices total:
Suppose you’re buying groceries. You have four items in your cart that cost $1.99, $4.93, $6.03, and $5.14.
If all you have is $20 in your wallet, is that enough to pay for the items?
I think that’s a very realistic question.
It would take you at least a little bit of time to add up those numbers individually and get an exact number. Would it answer your question? Absolutely. But you don’t need an exact answer.
The smarter thing to do would be to simply round the numbers. We should be saying to ourselves, “2 + 5 + 6 + 5 equals 18… throw in some tax… and I should still be under $20.”