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.

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.

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

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

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 4mulaFun, The Teacher Studio, Teaching 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.

### Angela Culley

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Nice great article.

The really nice tricks for mental math.

Very nice tricks Real Life Math everyone learning about it.

Nice. Now I’ll be able to calculate a tip mentally.

Shaun Hoobler recently posted…app dev empire review

Thanks Jamie! I love this monthly linkup!

Great post! I do exactly the same thing… round numbers!!! Wooo!

Thanks so much for linking up! We do hope you can join us again in the future!

Jamie aka MissMathDork