There are 30 days in April.
If 18 days had rain, it means 12 didn't.
18/30=x/100
Make sure both fractions can be made into fractions with a denominator of 100:
180/300=x/100
Divide 180/300 by 3:
60/100=x/100
This shows that the 18 days that did rain take up 60% of the whole month.
So then the remaining 12 days that didn't have rain must take up 40% of the month.
Solution:
40% of the days did not have rain during the month of April