You ended without giving the values, so we can't get the exact answer. However, I can tell you steps.
First, you need to find the z-score. Then, look up the z-score on a normal distribution table and you will get the percent.
Let's call your value X. Just plug X into the equation below and you will have your z-score.
(x - 20.5) / 3.5 = z
Now, look up the z-score and you will have your probability.