70% of the days in june were sunny. how many days were sunny?

Your answer will be 30% my trick is subtract 100% from 70%
 

There are 30 days in June. So 30 days represent 100% of the days. We have:

30 100%
  x  70%  100x= 2100 x= 21

Additionally, you can do to know the percentage of a number:  30 multiplied by 0.7 you will have 21. 0.7 means you are calculating 70% of the number you are multiplying.