First: work out the difference (decrease) between the two numbers you are comparing. Then divide the decrease by the original number and multiply the answer by 100. If your answer is a negative number, then this is a percentage increase. :)
It depends on the questions, some times you just substract the number from 100 which is the full percentage but sometimes it is like the example below.
lets say a city lost 2% of population each year population now?
