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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Hey there

Angela ,

Wonderful post! We are linking to this great article on our blog site from your *MATH NINJA* Open Web. You’ve outdone yourself this time by writing on the topic of Calculate A Tip Mentally. Keep rockin’ it..

I would like to share a few tricks which i found cool.

Two digit number and its reverse always divisble by 11

The sum of a two digit number and its reverse is always divisible by 11

eg (66+66/11 =132/11=12)

89+98 /11=17 etc etc)

Magic of 37

any 3 digit number where all the digits are same is divisible 37.yes!! 333,444,555,666…all are divisible by 37

Mental Multiplication with 11

multiplication by 11,for eg 35*11=385(3(3+5)5) keep the first and the last digit same and the middle digit is the sum of the two

multiplication of numbers with same tens digit

23*27=621.

step1: 2*(2+1)=6 is the first digit.

step2: 7*3=21 second digit

multiply numbers between 11 and 19.

eg) 18*14=252.

step 1: 18+4=22

now add a zero at the end we get 220

step 2 :multiply the right most digits 8*4=32

Add 220 and 32

we get 220+32=ans

The sum of 5 successive whole numbers is always divisible by 5

eg) 6+7+8+9+10=40 which is divisible by 5

The product of 3 consecutive natural numbers is always divisible by 6

eg) 6*7*8=336/6=56

The product of 3 consecutive natural numbers is always divisible by 24(if the first of the 3 natural consecutive natural numbers is even)

8*9*10=720/24=30

The last digit of the product of any 9 consecutive natural numbers is always 0

eg )(1*2*3*4*5*6*7*8*9=362880)

(as the product of any 5 consecutive natural numbers is always 0)

mentally squaring for 5

eg) 35*35=3*(3+1)…..25=1225

It was really insightful.

Thanks for the info.

Wanna have more contents from you.

Cheers

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