I hate our modern school system. Nobody teaches mental math anymore.
Caveat: Remember 15% is itself an approximation but quite accurate enough for light airplane speeds.
Here's the "trick" to 15% in your head.
Looks daunting kinda, right?
Try it this way...
15% is the same thing as 10% + 5%.
Oh!
Starting number 60 knots...
10% is 6. Half of that (5%) is 3.
6+3= 9
Add back to original number of 60.
69.
Easy to do in your head.
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100
10% is 10. Plus half of 10.
10+5. 15.
Add back to original 100... 115 MPH.
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Play with it.
Oh and by the way... If you suck at calculating 15%, for tipping... Just pay 20%. Your server is making a whopping $3.35/hr anyway.
Plus 20% is easier even. Take 10% and double it.
Bill is $1264.57 for your crazy night on the town.
1264.5 ... Round up you cheapskate... 1265
1265 * 2 looks hard. Let's break it up a bit.
1200 * 2 = 2400
65 * 2 = 130
$2530 is your tip there, big spender.
Warning: Judging by that bar tab a) They've already assessed an 18% gratuity. Read the receipt. b) You're drunk. That's why they did it.
Catch a cab and go home.
Now, if you say you need a metal landing calculator...