You'll need to calculate the applicable z-score:
(given value of x) - (mean value)
z = -----------------------------------------------
standard deviation
370-400 -30
Here, z = --------------- = ---------- = -0.3
100 100
Use either a table of z scores or your calculator to determine the area under the standard normal curve to the left of z = -0.3.
According to my TI-83 Plus calculator, this area is 0.382.
The probability of selecting at random a score less than or equal to 370 is 0.382.