Question: how to calculate probability with standard deviation?
Answer: We know that the probability P(X > 75) is equal to 1 – P(X ≤ 75), so we can use a table to find P(X ≤ 75). ... 
P(Z ≤ 0.5) = 0.6915.
Now we can calculate P(X > 75).
P(X > 75) = 1 – P(X ≤ 75) = 1 – P(Z ≤ 0.5) = 1 – 0.6915 = 0.3085.