Explain how to calculate percentage decrease.

To calculate percentage decrease, subtract the new value from the original value, divide by the original value, then multiply by 100.

Let's break this down step-by-step. Imagine you have an original value and a new value, and you want to find out how much the value has decreased in percentage terms.

1. **Find the Decrease**: First, subtract the new value from the original value. This gives you the amount by which the value has decreased. For example, if the original value is 50 and the new value is 30, the decrease is 50 - 30 = 20.

2. **Divide by the Original Value**: Next, take the decrease you just calculated and divide it by the original value. Using our example, you would divide 20 by 50, which gives you 0.4.

3. **Convert to a Percentage**: Finally, to convert this decimal into a percentage, multiply by 100. So, 0.4 multiplied by 100 equals 40%. Therefore, the percentage decrease from 50 to 30 is 40%.

In formula terms, it looks like this:
\[ \text{Percentage Decrease} = \left( \frac{\text{Original Value} - \text{New Value}}{\text{Original Value}} \right) \times 100 \]

This method works for any values you have. Just remember: subtract, divide, and multiply by 100. It's a handy way to quickly see how much something has decreased in relative terms, which can be really useful in many real-life situations, like analysing sales drops or understanding discounts.