Theory
How do you calculate a new value after a decrease?
With a percentage decrease, the old value is 100%.
For example, if 15% is subtracted, 85% remains. This corresponds to a growth factor of 0,85.
Multiply the old value by that growth factor to find the new value.
- Determine the growth factorSubtract the percentage from 100% and then divide by 100.
- Multiply by the growth factorMultiply the old value by the growth factor that remains.
You use multiplication with equal groups.
A × B means that you take the same amount B, A times.Instead of adding again and again, you calculate the product all at once.
A value of 80 decreases by 15%. What is the new value?
- Determine the growth factor100% - 15% = 85%, so the growth factor is 0,85.
- Multiply by the growth factor80 × 0,85 = 68.