Sunday, January 10, 2016

LPT: How to Calculate a Tip in Your Head

These are quick math tricks to make certain day-to-day calculations much easier.  When you eat at a restaurant, you'll typically tip your server.  Here's how to figure that in your head without multiplying.

Let's say the bill is $12.34.


Calculating an Exact Tip


  • Calculate 10% of the bill.  Ten percent of a number is simply that number with the decimal place shifted left one spot.  So ten percent of 12.34 is just 1.234 (round to 1.23).

10% tip:

  • Tip is $1.23

15% tip:

  • Start with the 10% of $12.34: 1.23.
  • Now cut that number in half to make 0.615 (rounded to 0.62).  This is 5% of the bill.
  • Add 1.23 and 0.62 together to make 15%:
  • Tip is $1.85

20% tip:

  • Double the ten percent: 1.23 * 2
  • Tip is $2.46



Easier Approximate Tip


That said, I typically approximate using only the dollar value, because it's easier.
  • Round up to $13.  Calculate 10% of 13.  Ten percent of a number is simply that number with the decimal place shifted left one spot.  10% of 13 is 1.30

10% tip:

  • Tip is $1.30

15% tip:

  • Cut 1.30 in half to make 0.65.  This makes 5% of the bill.
  • Add 1.30 and 0.65 together to make 15%: (1.30 + 0.65)
  • Tip is $1.95

20% tip:
  • Double the ten percent: 1.30 * 2
  • Tip is $2.60

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