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I really enjoy spending time with friends over a shared meal.  Today I thought I would share the math I use to calculate a tip mentally at some of my favorite restaurants. Since I eat out a lot, I get almost daily practice calculating 15 – 20% of my total bill and sums to 100. Let me show you the mental math I used while calculating last night’s bill of $54.32.

Calculate a tip mentally

The first step I use isn’t necessary, but I like to enter whole numbers into my checkbook. The first thing I always do is determine the amount of change needed to get me to the next whole dollar. By doing this step, my change will end in zeros on my final amount, which will make it easy to add in my checkbook.

  • As soon as I see the 32 following the decimal, I immediately started looking for a number that combined with 32 would make 100. It is almost like I see the number line below in my head.

Number line

This,$0.68, is the amount of change I will use for my tip.

Calculate the change portion of a tip

The next step is to calculate 15% of the bill. To begin, I realize that 15% = 10% + 5%. This is a helpful fact because our number system is based on tens. I will calculate 10% and 5% and add each together.

  • 10% of my bill, $5.43, can be calculated by just moving the decimal point one place to the left.
  • 5% is half of 10% so I calculate 1/2 of $5.43 to find 5%.
  • Half of $5 is $2.50, half of 40 cents is 20 cents, and half of 3 cents is 1.5 cents. All together, 5% of my bill would be 2.5 + 0.4 + 0.015, which is rounded to $2.92.

My 15% tip should be $5.43 + $2.92.

  • 5 + 2 = 7
  • 0.92 + .08 (from the 0.43 cents) = 1
  • 0.43 – 0.08 = 0.35

Therefore, 15% is 7 + 1 + 0.35 = $8.35

Since I already chose the change portion of my tip, I am going to use $8 for the cash portion.

Full tip calculated

 

Now, calculating the total is easy, I know that the change adds up to a dollar. So I add 54 + 1 + 8 = 63.

total bill

Okay, so you are probably thinking thank goodness I have a calculator! Well this flexibility with numbers is just what our kids need, so hand over a pencil and a piece of paper to your child and ask them to calculate the tip from now on 🙂

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Today, I’m linking up with 4mulaFunThe Teacher StudioTeaching to Inspire in 5th, and MissMathDork for a look at how math is used in the real world.  You might want to check out their Pinterest board to see previous Math is Real Life posts.

Math is Real Life Link-up

Angela Culley

Angela Culley

Owner & Tutor at Math Ninja, LLC
As the founder of Math Ninja, Angela provides online and in person math tutoring for all ages including those studying for the GRE, GMAT, SAT, ACT, PPST, and other math specific subject assessments. Prior to launching her business, Angela coached K-12 educators on effective teaching strategies, curriculum writing, and assessment development.As a classroom teacher, she taught math to students ranging from grade 5 - 12. She has also taught both undergraduate and graduate math courses as an adjunct professor at the University of Virginia and Mountain State University.
Angela Culley
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