Calculating with percentages is a common and useful skill! Percentages represent parts per hundred, so 1% = 1/100 = 0.01. Here’s a quick guide to the most common calculations:
1. Finding a Percentage of a Number
Question: What is 20% of 50?
Calculation:
20% of 50 = (20/100) × 50 = 0.20 × 50 = 10
2. Finding What Percentage One Number Is of Another
Question: What percentage is 10 of 50?
Calculation:
(10 / 50) × 100 = 0.20 × 100 = 20%
3. Increasing or Decreasing a Number by a Percentage
Question: Increase 50 by 20%.
Calculation:
50 + (20% of 50) = 50 + 10 = 60
Question: Decrease 50 by 20%.
Calculation:
50 – (20% of 50) = 50 – 10 = 40
4. Finding the Original Number Before a Percentage Increase or Decrease
Question: A number increased by 20% is 60. What is the original number?
Calculation:
Let the original number be x.
x + 20% of x = 60
1.20x = 60
x = 60 / 1.20 = 50
5. Percentage Change
Question: What is the percentage increase from 40 to 50?
Calculation:
((50 – 40) / 40) × 100 = (10 / 40) × 100 = 25%
6. Reverse Percentage (Finding the Base)
Question: 25% of a number is 15. What is the number?
Calculation:
Let the number be x.
25% of x = 15
0.25x = 15
x = 15 / 0.25 = 60
