Vol: 15 edition xi
Do You Tip? Do you know the balance before the cashier gives the total?
Here is a quick and easy formula.
Who needs a calculator?
You want to leave a 15% tip on a meal that cost $32.00.
First, convert the 15% to an actual number that can be used in a calculation. For percent’s, this is always done by simply dividing the percent (in this case 15%) by 100%.So, the conversational term “15%” becomes 15% / 100% = 0.15 in terms of a real mathematical number.
Second, you need to find out what 15% of your $32.00 meal cost. This is always done by multiplying 0.15 by $32.00, or
0.15 x $32.00=$4.80. 
So, the amount of tip you are going to leave is $4.80.
This makes the total cost of your meal (to write on your charge slip or other payment)
$32.00 + $4.80 = $36.80. 
Quick tip calculation for 15% tips:
If you’re leaving the standard 15% tip, here’s a quick and easy way to calculate it in your head, at the table in the restaurant, without embarrassing yourself by scribbling all over the back of a receipt or getting out a calculator:
15% is 10% + 5% (or 0.15 = 0.1 + 0.05, dividing each percent by 100). Thinking about it this way is useful for two reasons. First, it’s easy to multiply any number by 0.1; just move the decimal point left one digit. For example, 75.00 x 0.1 = 7.50, or 346.43 x 0.1 = 34.64 (close enough). Second, 5% is exactly half of 10%. So here’s how this quick and easy method works:
Take the total cost of your meal, $32.00 and multiply it by 0.1 by sliding the decimal to the left one digit to get $3.20 (you can do this in your head at the table). $3.20 is 10% of your meal cost. But the proper tip to leave is 15% (you need to throw in another 5%).
The additional 5% is just half of the 10% amount. What is half of $3.20? About $1.60. (Finding half of a number is also something you can do in your head at the table.) So, the total tip to leave is the 10% amount PLUS the 5% amount (for a 15% tip) or $3.20 + $1.60 = $4.80
The general rule here is:

That’s your tip!