Standard Normal Distribution Findng A

Question

I have the following question and i am dumbfounded on how to find the a in my given question.

$$\sigma= 10000$$ $$\mu= 50000 $$ Find the monthly income which is exceeded by 10 % of employees.

I have $$ P(X=a) = 0.1$$ $$P(\frac{a-\mu/}{\sigma}) = 0.1$$

I am stuck at this part.

Answer 1

Look up $\Phi^{-1}(0.9)$ in the tables and you see $1.2816$ so this equals $\frac{a-\mu}{\sigma}$