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Finding Percentages and Values

There are a few various values that we like to find when dealing with Percentages. Click below to go directly to the one that you want to become an expert in!

Adding a Percent to a Value

Method for All Percent Problems!


Adding a Percent to a Value
You have $20, and an item at the store costs $15. The sales tax is 8%. Do you have enough to purchase it? How much more do you need, or how much will you have left over?

This is a standard problem for math classes and is also used a lot in real life. Here's how we do it:

First, we want to convert the percentage to a decimal:
8% ÷ 100 = .08

Next, we multiply the percent (in the decimal format) by the amount:
15 * .08 = 1.2
The value 1.2 tells us that "8 percent of $15 is $1.2."

Last, we add the amount of the item to the amount that 8% is (1.2):
15 + 1.2 = $16.20.

To answer the question: Yes, you have enough to buy the item, because you have $20 and the item is only $16.20 with tax. And you will have $3.80 left over ($20 - $16.20)!


Method for All Percent Problems
The above method for adding percentages will always work, but there is a better way that works for any kind of percent problem. If you remember this, you will be able to do any problem!

The formula: IS/OF x Percentage/100

For example, if a problem reads: What is 15 percent of 80?

The problem works out to be:

We use 'X' since it is the only value we do not know.

Next, since we are dealing with percents expressed as ratios, we cross multiply, then divide. We only cross multiply with two values - never with the unknown value. Thus, we will multiply 15 and 100, giving us 1500.

We still have the "80" left over, along with the unknown "X." To finish this problem, we divide 1500 by 80:

1500 ÷ 80 = 18.75.

And that is our answer! We can write it as a sentence from the original question: "18.75 is 15 percent of 80."


Find another math lesson at the Math Lesson and Tutorial Center!

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