If we assume we have a normal distribution, we have the following formula.
mean + Z* standard deviation
Z is the Z score for the top 10%, so 90% or .90 fall below this score. From any Z table for the areas of a standard normal distribution, you find that Z = 1.28
Therefore the value on the test a person must score to be in the top 10% is
300 + 1.28(30)
300 + 38.4 = 338.4