Step by step
The question
Question: A book costs $20.
The sales tax is 5%.
What is the total cost?
1 Find the tax
Percent means out of 100.
So 5% is 5 divided by 100: 0.05.
The point moves 2 places to the left.
5% = 5 ÷ 100 = 0.05
The tax is 5% of $20.
"Of" means times: 0.05 times $20.
That is $1.
tax = 0.05 × $20 = $1
To multiply decimals: first 5 times 20, as whole numbers.
That is 100.
Then count the decimal places: 2.
So it is 1.00.
5 × 20 = 100
2 decimal places: 1.00
Careful!
5% is not $5.
It is 5% of the price.
Careful: not $20 + $5 = $25 ✗
2 Add it on
The tax is added to the price.
$20 plus $1 is $21.
total = $20 + $1 = $21
So the total is $21.
3 Check
Let's check another way: we pay 100% plus 5%: 105% of the price.
1.05 times $20 is $21.
The same.
So the answer is right!
100% + 5% = 105%: 1.05 × $20 = $21
Let's look back.
First: find the tax.
Then: add it on.
Last: check.
Answer
total = $20 + $1 = $21