I just found a pretty amusing and useful article. It teaches you how to do seemingly complex math calculation in your head. No, this thelistuniverse’s article is not that simple math tricks that you’ve learned in the high school. I guarantee you’ve never seen some of it.
One example:
Calculate a Tip
If you need to leave a 15% tip, here is the easy way to do it. Work out 10% (divide the number by 10) - then add that number to half its value and you have your answer:
15% of $25 = (10% of 25) + ((10% of 25) / 2)
$2.50 + $1.25 = $3.75
thelistuniverse is a site that creates lists of many different things. For example:
* Top 10 Bizarre Curses
* Top 10 Wired Microphones Still Used On Stage
* Top 10 Depressing Scenes in Cinema
* 10 Rare Cloud Formations
* Top 10 Bad Male Actors
* 10 Films You Haven’t Seen But Should
* 15 Truly Bizarre Patents
* Top 10 Flight Upgrade Tips
* Ten Whatchamacallits And Their Real Names
* 10 Great Writers Who Died Young
So just like this blog, thelistuniverse is worth visiting after your lunch break 
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