In a population of men, their heights are normally distributed with a mean of 178 cm. If a man with a height of 184 cm is shorter than 20% of the population, determine the standard deviation of the population.

the answer is 8.3cm, but how do u do it with a graphic-calculator (TI series)?

if I use this formula, z=(x-mean)/standard deviation, the answer is 30, which is not the right answer, can anyone tell me why?

Thank you!