Percent Calculations
Calculating percentages, percent increase/decrease, and applied percent problems (tax, tip, discount, successive percent changes)
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Imagine you're playing your favorite video game, and you find a treasure chest that gives you a 50% health boost! Or maybe you're at the store buying a cool new skateboard, and there's a giant sign that says 20% OFF! Percents are everywhere in the real world, and they are definitely going to pop up on the ISEE! 🎮🛹
But what exactly is a percent? The word percent is actually a secret code. 'Per' means 'for every,' and 'cent' means 'one hundred' (just like there are 100 cents in a dollar or 100 years in a century). So, 50% just means 50 for every 100. If you have a giant pizza cut into 100 tiny slices (whoa, that's a lot of cutting!), 50% means you get 50 slices. 🍕
On the ISEE, you'll need to know how to find the percent of a number, figure out what percent one number is of another, and calculate percent changes (like when a price goes up or down). The secret trick is to remember that percents can always be turned into fractions or decimals to make the math way easier. For example, 25% is just , which simplifies to .
Let's learn some awesome shortcuts so you can crush these percent questions and feel 100% confident on test day! Remember, there is no penalty for guessing on the ISEE, so if a question looks tricky, use your best real-world logic and go for it!
Practice Questions
3 practice questions for ISEE Upper Level
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- First, find the amount of the decrease: . To find the percent decrease, divide the decrease by the original price: . Converting the fraction to a percent gives .
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- Calculate the value of the stock on Tuesday: a 20% increase on $40 is . Next, calculate the value on Wednesday: a 20% decrease on $48 is . Since $38.40 is less than $40, the quantity in Column B is greater. (Note: Increasing a number by a percent and then decreasing the new number by the same percent will always result in a net decrease).
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- Let be the original price. Since the shoes are 15% off, the sale price represents of the original price. This gives the equation . Dividing both sides by 0.85 gives . The original price was $80.00.
Tips & Strategies
- Memorize common percent-to-fraction conversions like , , and . It saves tons of time on the math sections!
- Use the '10% trick'. To find 10% of any number, just move the decimal point one spot to the left. For example, 10% of 450 is 45. Need 20%? Just double that 45 to get 90!
- For Quantitative Comparison questions, remember the magic rule: is always equal to . If you see this on the ISEE, you can pick (C) without doing any math!
Common Mistakes
- Watch out for percent change questions! A huge mistake is dividing the change by the NEW number instead of the ORIGINAL number. Always put the difference over the starting amount.
- Don't forget that 'percent' means 'out of 100'. If an ISEE question asks for of a number, make sure you multiply by or , not (which is actually !).
Frequently Asked Questions
Do I have to use fractions, or can I use decimals?
You can use whichever one you like best! Sometimes decimals are easier, but fractions are usually faster on the ISEE if you simplify them first.
What if I forget a formula during the test?
Don't panic! Remember that 'percent' just means 'cents in a dollar'. If you need 25% of 80, just think 'what is a quarter of 80?' Use your real-world logic!
Is there a penalty for guessing on the ISEE if I can't figure out the percent?
Nope! There is zero penalty for guessing on the ISEE. If a percent question has you totally stumped, eliminate any crazy answers, take your best guess from the remaining choices, and move on.
How can I quickly find 15% of a number?
Find 10% first by moving the decimal to the left once. Then cut that number in half to find 5%. Add those two numbers together and you have 15%!