First we make a fraction with the reduced weight as the numerator and the weight before the weight loss as the denominator. This fraction will be less than 1. Multiply the fraction by 100. This will give you a number less than 100. Subtract this result from 100 to give you the percentage weight loss.
Example: 60kg reduced to 54kg. So we have 54/60=0.9. Multiply this by 100 to get 90%. Take it away from 100% and we get 10%, which is the percentage weight loss.