In order to calculate the increase we simply need to subtract the first number (18) from the second (22).
22 - 18 = 4
The difference is 4 - or in other words - this year it rained 4 more inches than last year. It's important to always subtract the numbers in this order. If your second number is lower than the first, you will have a negative number, and you'll be calculating percentage decrease instead of increase. Because the difference is positive we are calculating increase.
Finding the increase was easy. What we really want to know is the percentage change - that is what percentage of the original number is the difference. In the terms of our problem - last year it rained 18" and we want to know what percentage of 18 is 4.
4 / 18 = 0.222
This will give us the decimal solution, but since we are calculating the percentage we will need to multiple the answer by 100:
0.222 x 100 = 22.2
So the solution is 22.2%. In the terms of our problem, this year there has been a 22.2% increase in rainfall compared to last year.