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
4 practice questions for ISEE Middle Level
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- A 25% discount means the sale price is of the original price: . The employee discount takes an additional 20% off the sale price, meaning the employee pays of the sale price. Calculate 80% of $30: . The final price is $24.
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- An increase of 175% means the club gained new students. The total number of students is the original number plus the increase: . Alternatively, the new total is of the original amount: .
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- For Column A, a single 40% discount means the buyer pays 60% of the original price: . For Column B, the first 20% discount reduces the price to 80% of $400, which is . The second 20% discount reduces that price to 80% of $320, which is . Since $256 is greater than $240, the final price in Column B is greater.
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- First, find the total number of oak trees. Since 60% of the 150 trees are oaks, there are oak trees. Next, find 30% of those 90 oak trees to determine how many are infected: . There are 27 infected oak trees.
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%!