The participants for a program make a test, that has been made such that the degrees have the normal distribution mith mean value $300$ and standard deviation $60$. The program accepts $20\%$ of the participants with the highest degree at the test. Which is the minimum degree that is accepted?
So do we have to calculate the interval $[\mu -z\sigma, \mu +z\sigma]=[300 -60z, 300 +60z]$ so that it corresponds to $2\cdot 20\%=40\%$ ?
You need $60\%$ in that interval so that the tails have $20\%$ each. This is because of the symmetry of the normal distribution.