Probability, when i know the mean and std. dev

Hi. I have a normal distribution, where the mean is 900.000 and the standard deviation is 150.000.
The first question is, i need to find the probability that the value is under 500.000 in a given summer month, and i've got the result 0,38% with the help of a statistical software named JMP.

The second question is i need to find the standard deviation, when the probability is 5%. I don't really know how to find the solution
The answer to the first question you gave is correct: P(x < 500.000) = 0,38%.

To answer the second one, ask yourself two questions:
(1) What is the standard z-score for P = 5%?
(2) Given the mean, a sample value, and a standard z-score, how does one calculate the standard deviation?

You can use this online normal distribution calculator to do all of the calculations.