20% é a porcentagem

How Do You Calculate 20% Tip Without a Calculator?

To calculate a $20$ percent tip without a calculator, simply follow these steps:

1. Find $10$ percent ($\%$) of your total amount. You can do this by dividing the amount by 10, or moving the decimal point one place to the left!
2. Multiply the number you just got by $2.$ Think about it: $10\% \cdot 2 = 20\%.$
3. Round your answer to the previous step to the nearest hundredths place.

A Practical Note

A couple of things to keep in mind with these problems.

-The tip percentage won't always be $20$ percent. As a result, the tip amount won't always be the same, either. For especially good service, you might tip more.

-Taxes are also usually given as percents. You can use the same procedure to find the tax amount.

-Leaving a tip is only customary in certain countries.

-The tip amount used also depends on the number of people you're with.

Let's do another example!

Step One: Find $10\%$

To find $10\%$ of $54.89$, we simply move the decimal point one place the left.

Step Two: Multiply this Amount by $2$

$$5.489 \cdot 2 = 10.978$$

Step Three: Round to the Nearest Hundredth

$$10.978 \approx 10.98$$

So a $20\%$ tip for a dinner total of $\$54.89$ is $\boxed{\$10.98}$.

One way to find $20\%$ of a number is to use the technique for finding $x$ percent of a number, where $x = 20$. But, there's actually a trick that makes it easier. The trick has two steps:

1. Find $10\%$ of the number by moving the decimal point.
2. Multiply that number by $2$.

Step 1: Finding $10$%.

First: you can always find $10\%$ of a number by dividing that number by $10$ (because $10\% =\frac{10}{100} = \frac{1}{10}$). When you divide by $10$, you'll always be shifting the digits in your number down by one place value; in other words, moving the decimal point one step to the left so that the old tens place is now the ones place, the hundreds place is now the tens place, and so on.

In other words: to find $10\%$ of a number, just shift the decimal point one place to the left.

Step 2: Multiply it by $2$.

Once you have a value for $10\%$, all you have to do is multiply it by $2$ to get $20\%$, since $10 \times 2 = \boxed{20}$. Simple as that!

Let's try it.

Other tips

Since bills aren't usually easy dollar amounts and instead have ugly amounts of change as part of them, the number you get for $10\%$ might be a little bit difficult to multiply by $2$. When I go to restaurants, I generally try to round the number I get for $10\%$ up to the nearest dollar amount, and then multiply by $2$. That means that you're both tipping fairly and you don't have to do tricky multiplication.

Of course, be careful not to use the rounding method on a math test. It's not very precise.

(Video) How to Calculate a Restaurant Tip

by eHow

This video by Mike Mitchell explains how to calculate a tip using simple mental math. Before long you'll be able to calculate the tip in your head!

$\large{\text{Summary}}$

To calculate a $20\%$ tip, the steps are:

1. Find your total bill. This number is usually given in decimal form. Then, move the decimal point one digit to the left.
2. Multiply the resulting number by $2$.
3. Add the result to the bill.

It's okay to round your numbers up at any point to make the calculation easier!

If you want to tip something other than $20\%$, you can adjust it based on the number you get after Step 2 (but really, y'all... just tip at least $20\%$).

Sometimes you'll want to calculate total sales tax. This amount will change for a given county and city. In this case the "total bill" above will be the price of your purchase without tax. Then you'll be given a percentage. This percentage is one of the sales tax rates. From these pieces of information you can calculate total sales tax. Finally, add this number to the untaxed purchase price. This grand total is how much money you're really spending.

If your meal cost $\$34.90$, you should pay a total of about $\$42.00$ to include a decent tip.

$$\begin{align} \$&34.90\\\ \color{gray}{\text{Round up: }} &35\color{red}{.}00\\\ \color{gray}{\text{Decimal point: }}&3\color{red}{.}500\\\ \color{gray}{\text{Double: }} &7.00\\\ \color{gray}{\text{Add: }} &35.00 + 7.00\\\ \color{gray}{\text{Total: }} &42.00 \end{align}$$