Find the ratio in lowest terms of the number of 18 rainy days to the total numbers of 90 days in the period answers

You have 18 rainy out of 90 total.  You need to simplify each term.
18 = 1, 2, 3, 6, 9, 18
90 = 1, 2, 3, 5, 6, 9, 10, 15, 18, 30, 45, 90
Now find the largest number that is common.  That would be 18.  Divide both numbers by 18
18/18 = 1
90/18 = 5

So the simplified ration is 1/5